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This move is permanently replaced by the last move used by the target. The copied move has the maximum PP for that move. It is possible to then re-learn Sketch, if a level-up move, through the use of Heart Scales and the Move Tutor. This move cannot be blocked by Protect or Detect. This move cannot be blocked by Substitute. Fails if the target has not made a move, if the user has Transformed, or if the move is Chatter, Sketch, Struggle, or any move the user knows.
Sketch, in itself, is absolutely atrocious, as Smeargle can rarely control what move Sketch will copy from the opponent, and the move Smeargle does copy will be, an overwhelming majority of the time, absolutely useless on Smeargle. However, Sketch is also the reason Smeargle could ever be used, as it turns a Pokemon with absolutely miserable stats into a very reliable Baton Pass user, with access to literally every boosting move in the entire game, or a nice hazard setter, with access to Stealth Rock, Spikes, and Toxic Spikes. Another move that has lead to the competitive success of Sketch is Spore, allowing Smeargle to incapacitate the opponent's lead and have some free turns to either set up boosts or lay down hazards.