RBY Mechanics Guide

By Plague von Karma. Art by Plague von Karma.


The first generation of Pokémon is infamous for its glitches and weird mechanics. These largely define the way the game is played, and, therefore, it's crucial to understand them if one aims to succeed. For Smogon, we usually simulate the international version of Pokémon Yellow on Pokémon Showdown!, which matters because there are even glitches unique to the Stadium and Japanese versions of Generation 1. To keep things simple for this guide, we will strictly be going over Smogon-applicable RBY mechanics; i.e., those found in the international version of Yellow.

To preface, there will be times where we discuss a concept known as a "desync." This is a term covering glitches in Link Battles that lead to them "desynchronizing," causing communication errors. This effectively causes players to engage in different battles from each other, eventually leading to the appearance of glitch Pokémon, glitch moves, and more. On battle simulators, these generally aren't implemented. Should they not be ignored outright, workarounds are used to "prevent" them from occurring, such as the Desync Clause Mod on Pokémon Showdown, which changes desync situations to result in move failure.

To make things easier to understand, we will divide things into two sections: Battle Mechanics and Move Mechanics.

Table of Contents

Part 1: Battle Mechanics

To understand how everything works, it's important to first understand how the battle system works. There's no Mega Evolution, Z-Moves, Dynamax, etc in RBY. Not only that, there aren't any Fairy-, Dark-, or Steel-types, nor are there any items or abilities, already making for a substantially different metagame. Let's get into the actual mechanics, though.

Special Stat


In RBY, Special Attack and Special Defense are merged into one, singular "Special" stat. Therefore, if Special is increased or decreased, this will improve your offensive and defensive capabilities concerning special attacks. This makes Amnesia act like two Calm Mind uses, making Pokémon like Slowbro scary foes to face. Psychic also becomes a particularly scary move, as it drops Special a third of the time, which is effectively a double debuff.

DVs and Stat Exp

It is possible to max every one of your Pokémon's stats, which makes for a much bulkier metagame. Generally, Pokémon like Chansey and Snorlax become much tougher foes to KO, often requiring a concentrated effort from multiple Pokémon to take down. This is a vital concept to grasp, especially in a world where there are no items and many moves have low Base Power.


DVs and Stat Exp are RBY and GSC's equivalent to IVs and EVs, capping at 15 and 65535, respectively. The HP DV is determined by all the other stats, with every instance of an odd DV increasing the HP DV as follows; +8 HP DVs for Attack having an odd DV, 4 for Defense, 2 for Speed, and 1 for Special, adding up to the maximum of 15. The other DVs are determined at random, but because a Pokémon's encounter rate is one variable in DV randomization, not all Pokémon can get max DVs in every stat. Stat Exp is gained through the foe's raw base stats and is applied upon leveling up rather than immediately, which can make training take a long time. There is no limit on how much can be invested into all stats unlike in modern generations, so you can cap every single one.

For any individual raw stat, Stat Exp and DVs combined will amount to the same as a modern Pokémon's stat with full EVs and IVs but without a nature. In high-level play, there is little incentive to selectively invest in any given stat, though this isn't to say it's useless. Tanking Attack to 1 DV and 0 Stat Exp for reducing confusion damage is somewhat relevant; you need 1 Attack DV to ensure the HP DV reaches 15 in this situation, as otherwise, it drops to 7. Tanking the Attack stat will cause the Pokémon to lose Struggle wars more frequently, though, so for Pokémon like Chansey, lowering Attack can be problematic. Reducing stats to prevent stat debuff overflows is relevant for powerhouses like Mewtwo as well: more on this later.

Critical Hits


Critical hit rates are based on a Pokémon's base Speed: the faster the Pokémon, the higher the chance. The following equation defines a Pokémon's regular critical hit rate: Base Speed * 100 / 512. Due to this formula, the fastest Pokémon of the generation, Electrode, gets a 27.3% critical hit rate. On the lower end of the spectrum, it gives Snorlax a 5.85% critical hit rate, similar to that of later generations. For a list containing each Pokémon's regular critical hit rate, go here.

The damage dealt by a critical hit is partially based on the Pokémon's level, and, at Level 100, it rounds out to about twice the damage; the multiplier is around 1.95x. Critical hits will ignore any stat changes from either Pokémon during damage calculation; ergo, any Attack or Special boosts from both Pokémon will be ignored. Consequently, a fast setup sweeper will have an inherent decrease in consistency, as a critical hit will end up lowering its damage output. This also means that Swords Dance will not boost Slash, and Amnesia will not boost Crabhammer, at least if they land a critical hit. With this rule in mind, critical hits will also ignore things like opposing Defense boosts, Reflect, and Light Screen, which is a good trade-off for Pokémon like Persian, and in this case, largely defines its niche. Make no mistake: critical hits are very powerful.

Slash, Crabhammer, Razor Leaf, and Karate Chop possess an effect that increases the user's critical hit rate by 8x, making the formula Base Speed * 100 / 64. Many Pokémon—even those with somewhat average base Speed—will end up achieving very high critical hit rates with this mechanic, making the moves sort of like Frost Breath in modern generations. However, the chance for a critical hit will cap out at 99.6% due to a glitch in how the game determines whether a critical hit has landed, meaning that these moves landing a critical hit is never guaranteed.

Focus Energy, due to a glitch, ends up decreasing the user's critical hit rate by 4x, making the formula Base Speed * 100 / 2048. Theoretically, it can be used to prevent critical hits from occurring, but it isn't worth the moveslot, so it doesn't see use in high-level play.

Type Chart Differences and Type-based Attack Categories

Since RBY is a generation prior to the physical / special split, attack categories are based on typing. For example, Normal-type moves like Hyper Beam are always physical, and Water-type moves like Clamp are always special. Ergo, Pokémon like Kabutops are hard-pressed to find good STAB moves, while others such as Golem act as they always do in later generations.

Here is a table showing the categories:

Physical Special
Normal Grass
Fighting Fire
Rock Water
Ground Ice
Ghost Electric
Flying Psychic
Poison Dragon*

* Dragon is considered Special, but, due to it only having Dragon Rage, it doesn't actually see use.

On top of this, there are some type chart differences.

The Dark, Steel, and Fairy types are not present either. As a result of this, Magneton is mono-Electric, Clefable and Wigglytuff are mono-Normal along with their pre-evolutions, and Mr. Mime is mono-Psychic.

Same-type Status Immunity

An intended mechanic, as shown in Pokémon Stadium 2; if a Pokémon has the same type as a move used on it, any major statuses from move secondary effects are nullified. As a result of this mechanic, Electric-types cannot be paralyzed by Thunderbolt and Ice-types cannot be frozen by Ice Beam, for example. The only exception to this rule is Twineedle, which can still poison Bug-types. To add on to this, Poison-types cannot be poisoned full stop, not even by manually inducing poison with Toxic or Poison Powder, so Twineedle cannot poison Poison-types either. The big game-changer here, however, is that Normal-types cannot be paralyzed by Body Slam.

Stat Reapplication Glitch, Ignoring Stat Drops from Status, Stat Cap, and Stat Drop Overflow

The side effects of burn and paralysis—halving Attack and quartering Speed—can be reapplied if a Pokémon's stats are changed. For example, if Slowbro paralyzes Alakazam with Thunder Wave and then uses Amnesia, the Speed drop from paralysis will apply twice. Similarly, if Flareon burns a Pokémon and uses Tail Whip, their Defense will drop and their Attack will be halved twice.


Conversely, using moves that boost Attack or Speed can allow a Pokémon to ignore their stat drops from burn and paralysis, respectively. For example, if Kingler is burned and uses Swords Dance, its Attack will increase, ignoring the halving effect from the burn while still taking damage. Or, in the case of paralysis, Dragonite can use Agility to ignore the Speed drop while still having the chance to skip a turn.

All stats in RBY will cap at 999. In the case a move would boost past this, the game will set the stat to 999 if it was below 999, or make the move fail otherwise. For example, if a Venusaur with 298 Special were to use Growth to +5, it would cap the stat at 999 instead of letting it reach 1043. Because of the 999 stat cap, all further uses will fail, applying a "-1 deduction" on top of the move's existing stat stage change.


For moves that boost by one stage, like Meditate, the -1 deduction cancels out the move's boost as intended. In the case of moves that boost twice, such as Swords Dance, the -1 deduction will still apply. However, it went unnoticed that this gives a net gain of +1, allowing stat stages to "rise" once more, albeit still capped. Ergo, if a Pinsir were to raise its Attack to +4 using Swords Dance, which has the stat cap at 999, it can still technically go to +5 with no visible changes. It cannot, however, reach +6, as Swords Dance can only increase by +1 from +5, so the -1 deduction actually works. This, in most cases, is irrelevant to gameplay.


It is possible for stats that "rose" beyond the 999 stat cap to overflow if a debuff is sustained. The cap doesn't actually get applied when debuffs occur, and since the damage calculation is incapable of handling stats beyond 1023, debuffs will lead to an overflow. Let's say Mewtwo reaches +5 Special, or 1421 without the stat cap. If it sustains a debuff from something like Psychic, this Special will overflow into an extremely low number, and if you consider the Special stat serving as an offensive and defensive stat, you can imagine how comical it is to watch even Absorb OHKO it.


If a Pokémon is frozen, it cannot be thawed without an item, being hit by Haze, or being hit with a move that can burn. The latter means that Ember, Fire Punch, Flamethrower, and Fire Blast are all valid moves to thaw a foe. As a result, if you're frozen, it is completely on the opponent to thaw you, and most competitive teams don't use moves that can do this. This makes the freeze status a pseudo-KO. It should be noted that Fire Spin, due to not having a burn chance, cannot thaw foes.

However, in the case a Pokémon is thawed by a faster Pokémon and tries to use a move, this can cause a desync. On one side, the Pokémon will use the move they chose in the previous turn, while on the other, a glitch move will be used due to the game skipping the "FIGHT" menu.

For a far more detailed analysis, look here.



Sleep can last up to 6 turns, including a '0 turn sleep' where the target Pokémon wakes up the same turn it was put to sleep. Sleep turns are determined upon the status being inflicted; therefore, there is, effectively, a 1/7 chance of it waking up on each turn up until turn 6. This makes sleep a very powerful status, as it can definitely be equivalent to KOing a Pokémon in some circumstances. Furthermore, a Pokémon cannot act on the turn it wakes up, so it's possible to repeatedly put a Pokémon to sleep if the user is faster. As such, it isn't uncommon to see Pokémon like Gengar trying to land their sleep moves for extended periods of time.

If a Pokémon has used Hyper Beam the same turn prior to a sleep move's use, the sleep move will not only skip accuracy checks but also ignore any status when putting the opposing Pokémon to sleep. As a result, this is the only circumstance in which it's possible to status an already-statused Pokémon.


Confusion damage uses the Pokémon's raw Attack stat, dealing typeless, physical damage. Confusion lasts 1-4 turns and there is a 50% chance the Pokémon will attack itself. Considering the odds, there is a 76.6% chance the Pokémon will attack itself at least once. Confusion damage will ignore Reflect and has no damage variance. However, it will consider stat boosts; this means Swords Dance will increase the damage and Acid Armor will decrease it. Considering burns halve your Pokémon's Attack, being burned will reduce the damage it takes.

Decreasing investment in Attack will influence confusion damage, as it uses the raw stat. With the Attack tanking concept from the stat section in mind, Alakazam can decrease the damage it takes from 37 (11.8%) to 21 (6.8%), almost halving it. When damage is taken from confusion and either Pokemon has a Substitute, the game will attempt to deal damage to the opposing Pokémon's Substitute, regardless of whether it actually has one. As a result, a Pokémon's own Substitute is technically immune to its own confusion damage. However, it can break as a result of the opponent's.

Here is a table showing the regular damage a Pokémon will take from confusion:

Pokémon 1 Atk DV + 0 Stat Exp Dmg Max Stat Dmg Min % Max %
Bulbasaur 20 35 6.8% 11.9%
Ivysaur 21 35 6.5% 10.8%
Venusaur 23 35 6.3% 9.6%
Charmander 22 38 7.8% 13.5%
Charmeleon 23 37 7.2% 11.6%
Charizard 25 37 7.0% 10.3%
Squirtle 17 30 5.8% 10.3%
Wartortle 19 31 5.9% 9.7%
Blastoise 21 31 5.8% 8.6%
Caterpie 15 33 5.1% 11.3%
Metapod 9 24 3.0% 7.9%
Butterfree 18 33 5.6% 10.2%
Weedle 18 37 6.4% 13.1%
Kakuna 11 27 3.8% 9.2%
Beedrill 33 50 9.9% 15.0%
Pidgey 20 37 7.1% 13.1%
Pidgeotto 22 37 6.7% 11.2%
Pidgeot 24 36 6.5% 9.8%
Rattata 25 44 9.5% 16.7%
Raticate 28 42 8.9% 13.4%
Spearow 29 48 10.2% 17.0%
Fearow 29 42 8.7% 12.6%
Ekans 24 41 8.8% 15.0%
Arbok 27 40 8.4% 12.4%
Pikachu 26 46 9.5% 16.8%
Raichu 32 46 9.9% 14.2%
Sandshrew 21 33 6.9% 10.9%
Sandslash 23 33 6.5% 9.3%
Nidoran-F 18 33 5.8% 10.5%
Nidorina 20 34 5.8% 9.9%
Nidoqueen 23 34 6.0% 8.9%
Nidoran-M 24 42 8.1% 14.2%
Nidorino 25 40 7.7% 12.3%
Nidoking 27 39 7.4% 10.7%
Clefairy 18 34 5.2% 9.9%
Clefable 22 34 5.6% 8.7%
Vulpix 18 35 6.5% 12.5%
Ninetales 23 35 6.6% 10.0%
Jigglypuff 25 47 5.8% 10.9%
Wigglytuff 28 44 5.8% 9.1%
Zubat 21 39 7.4% 13.8%
Golbat 25 38 7.1% 10.8%
Oddish 19 33 6.5% 11.3%
Gloom 21 34 6.5% 10.5%
Vileplume 22 34 6.2% 9.6%
Paras 25 40 9.2% 14.7%
Parasect 27 39 8.4% 12.1%
Venonat 21 37 6.5% 11.5%
Venomoth 23 37 6.7% 10.8%
Diglett 28 49 12.6% 22.0%
Dugtrio 30 45 11.0% 16.5%
Meowth 21 39 7.4% 13.8%
Persian 24 38 7.2% 11.4%
Psyduck 21 36 6.9% 11.9%
Golduck 24 36 6.6% 9.9%
Mankey 35 53 12.4% 18.7%
Primeape 35 49 10.5% 14.7%
Growlithe 28 44 8.9% 14.1%
Arcanine 31 43 8.1% 11.2%
Poliwag 22 39 7.8% 13.8%
Poliwhirl 22 35 6.6% 10.5%
Poliwrath 22 33 5.7% 8.6%
Abra 14 38 5.5% 15.0%
Kadabra 18 37 6.4% 13.1%
Alakazam 21 37 6.7% 11.8%
Machop 30 45 8.7% 13.1%
Machoke 31 44 8.5% 12.1%
Machamp 36 48 9.4% 12.5%
Bellsprout 33 51 10.9% 16.8%
Weepinbell 33 49 9.9% 14.7%
Victreebel 33 47 9.1% 12.9%
Tentacool 19 37 6.7% 13.1%
Tentacruel 23 37 6.3% 10.2%
Geodude 20 31 7.1% 11.0%
Graveler 22 31 7.0% 9.9%
Golem 23 31 6.3% 8.5%
Ponyta 30 45 9.9% 14.9%
Rapidash 31 44 9.3% 13.2%
Slowpoke 22 35 5.7% 9.1%
Slowbro 18 28 4.6% 7.1%
Magnemite 12 25 4.7% 9.9%
Magneton 16 27 5.3% 8.9%
Farfetch'd 24 38 7.8% 12.4%
Doduo 33 49 12.1% 17.9%
Dodrio 34 46 10.5% 14.2%
Seel 17 32 5.1% 9.6%
Dewgong 21 32 5.5% 8.4%
Grimer 30 45 8.3% 12.4%
Muk 31 43 7.5% 10.4%
Shellder 17 27 6.5% 10.3%
Cloyster 16 23 5.3% 7.6%
Gastly 18 37 6.8% 14.1%
Haunter 21 37 7.2% 12.6%
Gengar 23 37 7.1% 11.5%
Onix 9 17 3.3% 6.2%
Drowzee 20 36 6.2% 11.1%
Hypno 23 36 6.2% 9.7%
Krabby 28 39 10.6% 14.8%
Kingler 29 38 9.3% 12.1%
Voltorb 13 28 4.6% 9.9%
Electrode 17 29 5.3% 9.0%
Exeggcute 13 25 4.0% 7.7%
Exeggutor 26 38 6.6% 9.7%
Cubone 14 25 4.6% 8.3%
Marowak 19 29 5.9% 9.0%
Hitmonlee 42 57 13.9% 18.8%
Hitmonchan 30 42 9.9% 13.9%
Lickitung 17 30 4.4% 7.8%
Koffing 17 28 6.0% 9.9%
Weezing 20 29 6.0% 8.7%
Rhyhorn 22 33 6.1% 9.1%
Rhydon 28 37 6.8% 9.0%
Chansey 7 35 1.0% 5.0%
Tangela 13 23 3.9% 6.9%
Kangaskhan 27 39 6.5% 9.4%
Horsea 14 27 5.3% 10.3%
Seadra 17 28 5.4% 8.9%
Goldeen 23 37 7.8% 12.6%
Seaking 30 43 8.3% 11.8%
Staryu 17 32 6.5% 12.2%
Starmie 21 33 6.5% 10.2%
Mr. Mime 16 29 5.7% 10.2%
Scyther 31 43 9.0% 12.5%
Jynx 23 41 6.9% 12.3%
Electabuzz 29 43 8.7% 12.9%
Magmar 33 47 9.9% 14.1%
Pinsir 30 41 9.0% 12.3%
Tauros 26 36 7.4% 10.2%
Magikarp 6 21 2.5% 8.6%
Gyarados 35 47 8.9% 12.0%
Lapras 25 36 5.4% 7.8%
Ditto 19 35 6.4% 11.7%
Eevee 21 37 6.7% 11.8%
Vaporeon 23 37 5.0% 8.0%
Jolteon 23 37 6.9% 11.1%
Flareon 43 57 12.9% 17.1%
Porygon 19 32 5.7% 9.6%
Omanyte 11 22 4.0% 8.1%
Omastar 14 23 4.1% 6.7%
Kabuto 22 33 8.4% 12.5%
Kabutops 27 37 8.4% 11.5%
Aerodactyl 33 47 9.1% 12.9%
Snorlax 35 48 6.7% 9.2%
Articuno 21 32 5.5% 8.4%
Zapdos 25 36 6.5% 9.4%
Moltres 27 38 7.0% 9.9%
Dratini 26 42 9.1% 14.7%
Dragonair 27 41 8.3% 12.6%
Dragonite 34 44 8.8% 11.4%
Mewtwo 29 40 7.0% 9.6%
Mew 25 35 6.2% 8.7%

Back to top of table!

Paralysis, Burn, and Poison

Unlike in later generations, paralysis lowers Speed by 75% and burns directly halve Attack. Bad poison inflicted by Toxic is a secondary status that gradually increases residual damage and will be removed if the afflicted switches out, replacing it with normal poison and resetting the counter. If the afflicted KOes a Pokémon, however, residual damage does not apply.

All status-based damage—poison, burns, and Leech Seed—uses a counter that begins at T/16, where T is the number of turns that have passed since bad poison began. From here on out, we will call this the "Toxic counter." This counter stays at 1 if it hasn't been put into effect. Therefore, all status-based residual damage begins at 6.25% and will not change unless Toxic has connected with a Pokémon. If Toxic is in effect, T will increase every turn. However, due to Leech Seed sharing the Toxic counter, being afflicted with both Leech Seed and Toxic will effectively double the counter every turn and KO a Pokémon in just 5 turns. Rest will not reset the counter, and if a Pokémon is burned after Rest removes bad poison, this will cause a "badly burned" effect, causing the Toxic counter to compound burn damage as if it were poison. Haze will reset the Toxic counter upon use.

Turn End Mechanics

If a Pokémon faints, the turn ends there and then. Therefore, any end-of-turn effects—such as Hyper Beam forcing a recharge turn or a Pokémon sustaining residual damage—are skipped.

Multi-hit Damage Calculation

All "multi-hits" will deal the same amount of damage, unaffected by any damage variance. This counts for the hits of typical multi-hit moves like Fury Attack as well as the hits of partial trapping moves like Wrap—which are "attacks" that depend on the user's stats and not set percentage damage like in later generations. Thus, if a critical hit connects, all the multi-hits will effectively critically hit. If a multi-hit move has a status effect, it will apply strictly on the last hit. Ergo, only Twineedle's last hit has a chance to inflict poison.

Partial Trapping

RBY partial trapping should be treated like a completely separate entity to modern-day partial trapping. Partial trapping moves—Wrap, Bind, Clamp, and Fire Spin—will deal the same amount of damage every turn, similar to multi-hit moves. Additionally, a user will be locked into the move for 2-5 turns, and a target will be completely immobilized during the ordeal, unable to strike back. There is a 37.5% chance for 2 turns of trapping, a 37.5% chance for 3 turns, a 12.5% chance for 4 turns, and a 12.5% chance for 5 turns. In the event of a Pokémon with an immunity, such as Ghost-types against Wrap or Bind, they will be immobilized but will not take damage.

If the trapper switches, the trapped Pokémon will not be able to move, making moves like Wrap pseudo-pivot moves. On the flip side, a faster trapped Pokémon will get to punish the switch-in, as it will be freed the following turn. The trapped Pokémon can also switch out, regardless of Speed. Switching out while trapped will force the opposing trapper to "reuse" the move, restarting the 2-5 turn lock and subtracting one PP, giving a form of PP stall counterplay. If the move is forced to be reused through switching at 0 PP though, it will roll over to 63 (this will not occur if the opponent tries to use the move at 0 PP). With these factors considered, there is quite a bit of depth to partial trapping situations.


If a Pokémon has used Hyper Beam and the opposing partial trapping user misses, the recharge turn is skipped. If the partial trapping move is used and misses while the foe is recharging, though, the opposing Hyper Beam user will automatically reuse Hyper Beam. If they have 0 Hyper Beam PP at this point, the move's PP will also roll over to 63. For example, if Tauros uses its last Hyper Beam and fails to KO a normal Speed Dragonite, then Dragonite uses Wrap and misses, Tauros can immediately move again next turn. If Dragonite uses Agility right after the final Hyper Beam and then uses Wrap next turn and misses, however, Tauros will immediately use Hyper Beam and get 63 more uses.

If a partial trapping move is copied by Mirror Move and succeeds, it works as normal. However, if the trapped Pokémon switches out in a Link Battle, due to Mirror Move forcing the move to fail on one side and having the user continue partial trapping on the other, this will cause a desync.

1/256 Accuracy and Effect Chance

No move in the chromatic generation is guaranteed to hit, except Bide and Swift, which skip accuracy checks, even against flying or underground foes. Every other move has an inherent 1/256 (~0.4%) chance to miss as a result of accuracy being checked between 0 and 256, with normal accuracy capped at 255. Only moves that target the opponent are subject to this.

Considering this uncertainty, it is possible to create a conversion table to show the true accuracy of a move.

Stated Accuracy True Accuracy True Accuracy as a Percentage* Notable Move(s)
100% 255/256 99.6% Many!
95% 242/256 94.5% Razor Leaf
90% 229/256 89.5% Hyper Beam, Blizzard, Rock Slide
85% 216/256 84.4% Fire Blast, Wrap, Toxic, Crabhammer, Pin Missile
80% 204/256 79.7% Hydro Pump, Submission
75% 191/256 74.6% Clamp, Lovely Kiss, Stun Spore, Bind
70% 178/256 69.5% Fire Spin, Thunder
60% 153/256 59.8% Hypnosis
55% 140/256 54.7% Sing, Supersonic
30% 76/256 29.7% Fissure, Horn Drill

*Rounded to the nearest tenth

Effect chance also suffers from a similar but inverted uncertainty; this inversion is due to the way the probability is hardcoded. This uncertainty can effectively increase the chance of a secondary effect occurring from an attack beyond the stated chance. For example, Fissure will connect 76/256 times, whereas Body Slam will paralyze a foe 77/256 times.

Stated Probability True Probability True Probability as a Percentage* Examples
40% 103/256 40.2% The chance that Sludge poisons the target.
33% 85/256 33.2% The chance that Psychic drops the targets Special stat.
30% 77/256 30.1% The chance that Body Slam paralyzes its target
20% 52/256 20.3% The chance that Twineedle will poison its target.
10% 26/256 10.2% The chance that Ice Beam will freeze its target.

*Rounded to the nearest tenth

Recovery Move Failure

If a Pokémon's max HP and current HP have a difference of either 255 or 511, recovery moves will fail due to only checking the second byte of the user's current HP.


Let's look at this in more detail. RBY logically makes recovery moves fail if the Pokémon is already at full HP, or specifically if Max HP - Current HP = 0, which also means that Rest cannot be used to cure status at full HP. This failure condition seems simple, but because the game only sees the second byte of current HP, things go awry when your HP is over 255, as 255 is the maximum amount that can be stored in a byte. In the event of an overflow, a carry flag is set to take 1 when calculating whether the recovery move should fail...to detrimental effect, as this leaves the second byte with 0 if you have 255 or 511 less than max HP. Let's use this example with Chansey, a notorious user of a recovery move in Soft-Boiled, after losing 511 HP. Due to only checking one byte in this scenario, Chansey's max HP in the game's eyes is 191. So when taking away the current HP and the 1 from the carry flag, if it's 192, in the game's eyes, Chansey's HP will be treated as if it were the maximum amount. Thus, the recovery move fails. Due to not having enough HP to even get above 255, Abra, Diglett, Magnemite, and Magikarp are unaffected by this glitch entirely.

Here is a table showing each Pokémon's "failure number." Chansey and Snorlax are the only Pokémon capable of reaching the HP required for the 511 number to be reached and have been omitted from the table itself.

511 for Chansey: 192 HP / 27.3%

511 for Snorlax: 12 HP / 2.3%

Pokémon HP - 255 HP - 255 as %
Bulbasaur 38 13.0%
Ivysaur 68 21.1%
Venusaur 108 29.8%
Charmander 26 9.3%
Charmeleon 64 20.1%
Charizard 104 29.0%
Squirtle 36 12.4%
Wartortle 66 20.6%
Blastoise 106 29.4%
Caterpie 38 13.0%
Metapod 48 15.8%
Butterfree 68 21.1%
Weedle 28 9.9%
Kakuna 38 13.0%
Beedrill 78 23.4%
Pidgey 28 9.9%
Pidgeotto 74 22.5%
Pidgeot 114 30.9%
Rattata 8 3.0%
Raticate 58 18.5%
Spearow 28 9.9%
Fearow 78 23.4%
Ekans 18 6.6%
Arbok 68 21.1%
Pikachu 18 6.6%
Raichu 68 21.1%
Sandshrew 48 15.8%
Sandslash 98 27.8%
Nidoran-F 58 18.5%
Nidorina 88 25.7%
Nidoqueen 128 33.4%
Nidoran-M 40 13.6%
Nidorino 70 21.5%
Nidoking 110 30.1%
Clefairy 88 25.7%
Clefable 138 35.1%
Vulpix 24 8.6%
Ninetales 94 26.9%
Jigglypuff 178 41.1%
Wigglytuff 228 47.2%
Zubat 28 9.9%
Golbat 98 27.8%
Oddish 38 13.%
Gloom 68 21.1%
Vileplume 98 27.8%
Paras 18 6.6%
Parasect 68 21.1%
Venonat 68 21.1%
Venomoth 88 25.7%
Diglett -32
Dugtrio 18 6.6%
Meowth 28 9.9%
Persian 78 23.4%
Psyduck 48 15.8%
Golduck 108 29.8%
Mankey 28 9.9%
Primeape 78 23.4%
Growlithe 58 18.5%
Arcanine 128 33.4%
Poliwag 28 9.9%
Poliwhirl 78 23.4%
Poliwrath 128 33.4%
Abra -2
Kadabra 28 9.9%
Alakazam 58 18.5%
Machop 88 25.7%
Machoke 108 29.8%
Machamp 128 33.4%
Bellsprout 48 15.8%
Weepinbell 78 23.4%
Victreebel 108 29.8%
Tentacool 28 9.9%
Tentacruel 108 29.8%
Geodude 28 9.9%
Graveler 58 18.5%
Golem 108 29.8%
Ponyta 48 15.8%
Rapidash 78 23.4%
Slowpoke 128 33.4%
Slowbro 138 35.1%
Magnemite -2
Magneton 48 15.8%
Farfetch'd 52 16.9%
Doduo 18 6.6%
Dodrio 68 21.1%
Seel 78 23.4%
Dewgong 128 33.4%
Grimer 108 29.8%
Muk 158 38.3%
Shellder 8 3.0%
Cloyster 48 15.8%
Gastly 8 3.0%
Haunter 38 13.0%
Gengar 68 21.1%
Onix 18 6.6%
Drowzee 68 21.1%
Hypno 118 31.6%
Krabby 8 3.0%
Kingler 58 18.5%
Voltorb 28 9.9%
Electrode 68 21.1%
Exeggcute 68 21.1%
Exeggutor 138 35.1%
Cubone 48 15.8%
Marowak 68 21.1%
Hitmonlee 48 15.8%
Hitmonchan 48 15.8%
Lickitung 128 33.4%
Koffing 28 9.9%
Weezing 78 23.4%
Rhyhorn 108 29.8%
Rhydon 158 38.3%
Chansey 448 63.7%
Tangela 78 23.4%
Kangaskhan 158 38.3%
Horsea 8 3.0%
Seadra 58 18.5%
Goldeen 38 13.0%
Seaking 108 29.8%
Staryu 8 3.0%
Starmie 68 21.1%
Mr. Mime 28 9.9%
Scyther 88 25.7%
Jynx 78 23.4%
Electabuzz 78 23.4%
Magmar 78 23.4%
Pinsir 78 23.4%
Tauros 98 27.8%
Magikarp -12
Gyarados 138 35.1%
Lapras 208 44.9%
Ditto 44 14.7%
Eevee 58 18.5%
Vaporeon 208 44.9%
Jolteon 78 23.4%
Flareon 78 23.4%
Porygon 78 23.4%
Omanyte 18 6.6%
Omastar 88 25.7%
Kabuto 8 3.0%
Kabutops 68 21.1%
Aerodactyl 108 29.8%
Snorlax 268 51.2%
Articuno 128 33.4%
Zapdos 128 33.4%
Moltres 128 33.4%
Dratini 30 10.5%
Dragonair 70 21.5%
Dragonite 130 33.8%
Mewtwo 160 38.6%
Mew 148 36.7%

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Damage Storing Mechanics

RBY stores the last damage that was dished out by either Pokémon. This is for Bide and Counter's use, which will cite it when necessary.

Most status moves will reset the damage last dealt; however, some moves don't. Moves that don't include: Confuse Ray, Conversion, Focus Energy, Glare, Haze, Leech Seed, Light Screen, Mimic, Mist, Poison Gas, Poison Powder, Recover, Reflect, Rest, Soft-Boiled, Splash, Stun Spore, Substitute, Supersonic, Teleport, Thunder Wave, Toxic, and Transform. This can lead to some unintended results, such as Bide stacking the damage that was dealt the previous turn.

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Part 2: Move Mechanics

With every passing generation, moves in Pokémon will receive revisions. RBY is the earliest generation, and as such, many moves have significant differences to the present day. Here, we will compile all the notable differences. Knowing these mechanics is pivotal to playing the game and teambuilding effectively.

As a general rule, moves that have the same stated accuracy as present generations will not be noted, despite the 1/256 uncertainty technically making them different. In addition, there are numerous moves that have only received Base Power, stated Accuracy, or PP changes, which will not be noted either. If a move has major mechanics changes, however, these differences will be noted for completion's sake.

Acid, Aurora Beam, Bubble, Bubble Beam, and Constrict

These moves all have a 33.2% chance of their effects occurring.


The Bide user is locked into charging for 2-3 turns before attacking, dealing double the total damage it received. This ignores accuracy checks and type immunity, and even hits when the foe is semi-invulnerable during Fly or Dig. Bide isn't entirely committal either; the user can switch out during the move if the situation becomes unfavorable. If confusion or paralysis interrupts Bide, the effect ends. Sleep, freeze, flinching, and partial trapping will pause—but don't stop—Bide.

There are ways that Bide's damage can be stacked. If the foe switches out, the last damage dealt to the user will be re-added to the total. Ergo, if a Pokémon attacks with Thunderbolt and switches to something else, it will be as if Thunderbolt hit the Bide user twice. Additionally, some moves will also compound Bide's damage due to not clearing the last damage dealt that turn. These include: Confuse Ray, Conversion, Focus Energy, Glare, Haze, Leech Seed, Light Screen, Mimic, Mist, Poison Gas, Poison Powder, Recover, Reflect, Rest, Soft-Boiled, Splash, Stun Spore, Substitute, Supersonic, Teleport, Thunder Wave, Toxic, and Transform.

If a foe is using Bide and your Pokémon faints before the user "unleashes energy," possible through residual damage such as Toxic poison or recoil, the damage stored by Bide is supposed to be cleared. However, specifically for the side that took residual damage, only the most significant byte of the Bide damage is cleared, which means that the Bide damage stored may still be any value up to 255. This disparity typically causes no PRNG differences, but if a Pokémon faints due to the damage difference from Bide, a desync will occur.


In addition to partial trapping mechanic differences, Bind has 74.6% accuracy.


The Dark-type does not exist, so Bite is Normal-type. It also only has a 10% chance to flinch, down from 30%.


Conversion changes the user's type to match the foe's, both primary and secondary, making it effectively the same move as Reflect Type in later games. It doesn't clear the last damage dealt, so it will allow Bide damage to stack.


Counter is fairly notorious for its complicated mechanics, but it isn't as bad as it seems in context. If a Pokémon was last hit by a Normal- or Fighting-type move and the last move's Base Power is more than 0, Counter activates, dealing double the damage dealt last. Counter has a Base Power of 1, unlike in later generations where it's not specified. Counter also ignores type immunity, so it can hit Ghost-types like Gengar.


Counter does have some odd interactions due to how damage is stored in the game. Since damage isn't cleared upon a Pokémon switching out, Counter can work against Normal- and Fighting-type moves that dealt damage the previous turn. If Snorlax uses Body Slam on a faster Pokémon and immediately switches out, if the foe uses Counter, it will counter the Body Slam from the previous turn. This also applies to things like Hyper Beam, where a Pokémon can switch in, take the hit, and counter the move the following turn while the foe recharges. This is because recharging doesn't clear the last damage dealt. For an even more comical example, it is possible to counter an Explosion if a Pokémon switches out the turn after, dealing massive damage and often getting a free KO. This can be applied to crash damage from High Jump Kick, but not recoil from moves like Double-Edge.

Counter is a source of multiple desyncs because of its activation requirements; the most notorious one involves how it interacts with full paralysis. One cartridge interprets Counter based on the move hovered over by the opponent, while the other interprets Counter based on the last move actually used. A common instance of this desync is if a move is selected and the user experiences full paralysis. For example, if Alakazam uses Psychic one turn and fully paralyzes as it tries to use Seismic Toss, Counter will fail on one cartridge and succeed on the other. Similar situations can occur if the opponent switches out after hovering over a move, as hovering over a move will update the memory addresses Counter is meant to use on their end.

Dig and Fly

Both Dig and Fly are affected by the semi-invulnerability glitch, a quirk so powerful that the moves are banned in almost all competitive RBY formats. It causes users to be affected by semi-invulnerability if the moves are interrupted mid-use until Fly or Dig are used successfully; this means confusion and paralysis can inadvertently cause the glitch to occur. Only Swift and Bide can hit a Pokémon during semi-invulnerability in RBY, making these moves the only real counterplay.


Disable has a paltry 54.7% accuracy and disables a random move in the foe's moveset for 0-7 turns. If Disable misses a Pokémon under the effect of Rage, it will cause the opposing Rage user's Attack to increase.

Dizzy Punch

Dizzy Punch has no added effect.


Double-Edge has 100 Base Power and only inflicts 1/4 of the damage dealt as recoil.


Earthquake does not hit Pokémon during Dig.

Explosion and Self-Destruct

Explosion has 170 Base Power and Self-Destruct has 130. They halve the foe's Defense during damage calculation, making their Base Powers effectively 340 and 260, respectively. Users of Explosion and Self-Destruct will not faint if using either against a Substitute. Due to the turn ending mechanics, these moves provide free switch-ins to your Pokémon after use, as the foe will not be able to move if they survived the attack.

Fire Blast

Fire Blast has a 30.1% chance to burn!

Fissure, Guillotine, and Horn Drill

These moves deal 65535 damage as fixed damage-type moves. In addition, they only work on foes slower than the user. Using a Speed-boosting move such as Agility will allow Pokémon like Seaking to be capable of OHKOing any Pokémon.



Glare has 74.6% accuracy and can affect Ghost-types, unlike in GSC and ADV.


Gust is Normal-type and does not connect against foes using Fly.



Haze is a very odd move. It will remove any and all volatile changes to both Pokémon; this includes setting stat changes to 0, removing stat reductions caused by status conditions, and secondary status. Essentially, it is a full reset to the conditions for both Pokémon. It will also turn bad poison inflicted by Toxic into normal poison and reset the Toxic counter.

A significant issue to note is that Haze will remove the major status condition for the foe only, even though it removes stat reductions from statuses for both. This means that it will remove paralysis, burn, poison, sleep, or freeze if the foe is inflicted with it. Due to this, it's possible to exploit Haze users by switching in a Pokémon crippled by status. This makes the move a rare sight in competitive play, though its distribution can also be considered a deciding factor.

If Haze is used against a Pokémon that has been put to sleep or frozen during Rage, Bide, thrashing about moves, or a move's charge turn, the victim will become permanently immobilized. This also happens to Hyper Beam users while recharging, but only if they're frozen.

Overall, Haze will remove:

High Jump Kick and Jump Kick

High Jump Kick and Jump Kick have 85 and 70 Base Power, respectively. Jump Kick also has 25 PP. Curiously, if the moves fail to connect, the user only takes 1 HP of crash damage. This residual damage can be countered by Counter.

Hyper Beam

Hyper Beam is extremely powerful, with the recharge turn being skipped if a Substitute or opposing Pokémon is KOed. This makes it a common sight.

Karate Chop

Karate Chop is Normal-type rather than Fighting-type.

Leech Seed

Leech Seed absorbs 6.25% of the foe's HP, rounded down but not less than 1. This can be increased through Toxic; refer to the status section for more information. Even if a Pokémon is KOed by other residual damage—such as confusion or recoil—the health draining from Leech Seed will still occur. However, if Leech Seed itself KOes the target, health will not be recovered due to turn ending mechanics. Additionally, damage dealt by Leech Seed is done after any other residual damage, such as Wrap, poison, and burn. If a Pokémon is hit by Leech Seed switching in, they will have their HP drained that turn, but only if their Speed is less than or equal to the Leech Seed user's.

Like later generations, Grass-types are not affected by Leech Seed and the seeded status is removed through switching out. However, Haze can also remove it.

Low Kick

Low Kick is a 50 Base Power move with 89.5% accuracy and a 30.1% chance to flinch.


Mimic will randomly copy a foe's move with no restrictions attached. Even if your Pokémon already has the same move, Mimic can still potentially copy it. This copied move will have the same maximum and current PP of Mimic.


Minimize raises evasion by one stage, making it virtually identical to Double Team. It also has 20 PP, which does make it a strict upgrade.

Mirror Move

Mirror Move interacts oddly with multi-turn moves, ignoring them as they charge. This means that if a Pokémon uses Body Slam and then Solar Beam, Mirror Move will copy Body Slam on the charge turn. Similar situations occur with paralysis, freezing, flinching, confusion, and recharging from Hyper Beam. If the foe switches out, Mirror Move will fail, which is part of what causes the desync mentioned in the partial trapping section.

Night Shade and Seismic Toss

Due to set damage mechanics, Night Shade and Seismic Toss will hit Normal- and Ghost-types, respectively.

Poison Sting

Poison Sting has a 20.3% chance to inflict poison.


Psychic has a 33.2% chance to drop Special by one stage.


Psywave is quite odd. Psywave deals damage between 1 and 1.5x the user's level, which at Level 100 will deal up to 150 damage. It also has a 79.7% chance to connect, unlike newer generations.

In a Link Battle, Psywave will have a lower bound of dealing 0 damage on one side, meaning that there is an inherent chance the game will desync, increasing as the level lowers. At Level 100, this is a 0.662% chance. The lowest possible level to use Psywave at is Level 3 with Jigglypuff, which has a 20% chance to desync the game. The game will also crash if Psywave is used by a Level 0, 1, or 171 Pokémon, though none of these can be achieved without hacking.

Here is a table featuring Psywave desync chances by level:

Level Desync Chance
2 25.000%
3 20.000%
4 14.286%
5 12.500%
6 10.000%
7 9.091%
8 7.692%
9 7.143%
10 6.250%
11 5.882%
12 5.263%
13 5.000%
14 4.545%
15 4.348%
16 4.000%
17 3.846%
18 3.571%
19 3.448%
20 3.226%
21 3.125%
22 2.941%
23 2.857%
24 2.703%
25 2.632%
26 2.500%
27 2.439%
28 2.326%
29 2.273%
30 2.174%
31 2.128%
32 2.041%
33 2.000%
34 1.923%
35 1.887%
36 1.818%
37 1.786%
38 1.724%
39 1.695%
40 1.639%
41 1.613%
42 1.563%
43 1.538%
44 1.493%
45 1.471%
46 1.429%
47 1.408%
48 1.370%
49 1.351%
50 1.316%
51 1.299%
52 1.266%
53 1.250%
54 1.220%
55 1.205%
56 1.176%
57 1.163%
58 1.136%
59 1.124%
60 1.099%
61 1.087%
62 1.064%
63 1.053%
64 1.031%
65 1.020%
66 1.000%
67 0.990%
68 0.971%
69 0.962%
70 0.943%
71 0.935%
72 0.917%
73 0.909%
74 0.893%
75 0.885%
76 0.870%
77 0.862%
78 0.847%
79 0.840%
80 0.826%
81 0.820%
82 0.806%
83 0.800%
84 0.787%
85 0.781%
86 0.769%
87 0.763%
88 0.752%
89 0.746%
90 0.735%
91 0.730%
92 0.719%
93 0.714%
94 0.704%
95 0.699%
96 0.690%
97 0.685%
98 0.676%
99 0.671%
100 0.662%

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Quick Attack

If a Pokémon is put to sleep or frozen on the turn Quick Attack is attempted to be used, its increased priority persists until the end of the turn after the user wakes up, is defrosted, or switches out.

Rage, Petal Dance, and Thrash

Thrash is a 90 Base Power move with 20 PP and Petal Dance has 70 Base Power and 20 PP. "Thrashing about" will persist for 3-4 turns, and always confuses the user upon ending naturally, resetting confusion if it's already confused. If not ended naturally, such as through paralysis, confusion is not sustained. Sleep, freeze, flinching, and partial trapping will only pause Thrash, though. Confusion being sustained is not mentioned, and will only be discovered upon selecting a move.

Rage, Petal Dance and Thrash will recalculate accuracy values every turn using current accuracy, but not to more than 255/256 or less than 1/256. With accuracy debuffs or evasion buffs, this can lead to their accuracy progressively decreasing, even if the foe doesn't further alter accuracy or evasion during the move's duration. For example, if a Thrash user goes against a target that has increased its evasion, accuracy will be read as 168, or 65.8% considering the 1/256 uncertainty. However, the next turn, accuracy will scale to 112, or 43.9%. This can theoretically lead to it dropping to 1 accuracy, but due to the length of time thrashing about moves last, this is a very rare occurrence. For this same reason, in-context, this effect is almost unnoticeable.

Rage works a bit differently. If Rage misses while the "thrashing about" state is active as a result of lowered accuracy or foe evasion, its accuracy drops to 0.4%. Contrary to popular belief, this does not happen as a result of the 1/256 uncertainty. If Disable misses during Rage, it will still raise Attack. Nothing will stop Rage once it starts: even being thawed from freezing will not change this. Rage will not increase Attack past +6 or the 999 stat cap.

Razor Wind, Sky Attack, Skull Bash, and Solar Beam

Razor Wind has 74.6% accuracy. Sky Attack has no additional effect upon use: no increased critical hit ratio or flinch chance. Skull Bash does not raise Defense upon use.

All of these moves will not count as the last move used until they actually execute, so if Mirror Move is used, it will fail if a move wasn't used prior; if a move was used, Mirror Move will copy that one. Partial trapping, sleep, freezing, and flinching will pause but not stop these moves.

Recover and Soft-Boiled

Recover notoriously has 20 PP, which, with a PP Max, becomes an eye-watering 32. Both moves are otherwise the same and are subject to the recovery failure glitch.

Reflect, Light Screen, Mist

Reflect and Light Screen are permanent upon use, acting sort of like Iron Defense or Amnesia. However, before you start chucking Light Screen on your Chansey, know that these effects only apply when receiving damage; Light Screen doesn't boost Special. Mist is also permanent, but it only stops moves that directly drop stats and not those with secondary effects. So, it'll block String Shot, but not Psychic Special drops.


Rest is subject to the recovery failure glitch. Rest will not remove stat drops inflicted by paralysis or burn, nor will it reset the Toxic counter. The Rest user will wake up after 1 turn of sleeping, which, due to the waking up mechanics, makes it effectively the same as later generations.

Rock Slide

Rock Slide has no additional effect; as such, the flinch chance does not exist.

Roar and Whirlwind

These moves have no in-battle effect: there is no phazing in RBY.

Sand Attack

Sand Attack is Normal-type.


Stomp's damage is not doubled against Minimize.



If your Pokémon has exactly 25% of its HP and uses Substitute, the user will self-KO. Substitutes don't block major status, Disable, or confusion. Substitute does, however, prevent flinching. Substitute blocks poison, so Poison Powder, Toxic, and Poison Gas will fail against it, and moves with added poison effects will never apply them. Substitutes can fade if exactly 255 damage is dealt, even if that is less than the usual 25%. Confusion damage is dealt to opposing Substitutes if one is available. If a multi-hit attack breaks a Substitute, the multi-hit attack will end. Pokémon that use Explosion or Self-Destruct will not faint against Substitute. Substitute does not make Counter fail while active, so if a Substitute is hit with a Normal- or Fighting-type move, Counter will still work and hit for the damage dealt.


Struggle is considered Normal-type and will not affect Ghost-types, making infinite battles between Ghost-types possible. It will also deal 1/2 of the damage dealt as recoil and can critically hit. Struggle is subject to accuracy checks, so it can miss with accuracy drops and is affected by the 1/256 uncertainty.


Thunder has 120 Base Power, has a 10.2% chance to inflict paralysis, and misses against foes using Fly.


Teleport does nothing in-battle.


Toxic has 84.4% accuracy and bad poison is treated as a kind of secondary status effect that is added to regular poison. For more information, view the Paralysis, Burn, and Poison section.


Transform does not fail against any other Pokémon, causing Ditto vs Ditto to be an infinite battle. The foe's status condition isn't copied, though Reflect and Light Screen will have their boosts applied to the user's new stats when it is hit by an attack. Since base Speed isn't copied, the Transform user's critical hit rate remains the same.

Tri Attack

Tri Attack has no additional effect, so it can't freeze, burn, or paralyze foes.

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