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Plus grants a 50% Special Attack boost in double or triple battles if on the field with a partner using Plus or Minus. This boost will not stack if there are multiple Pokemon with Plus or Minus on the field.
Plus could have been a good ability in doubles if there were better Pokemon with it. Klinklang and Plusle are horrible users, and Ampharos is not much better. Here's to hoping for better Plus Pokemon in Generation VI.
Found in Dream World