Pokémon Ruby and Sapphire introduced double battles, in which two Pokémon fight on each side at the same time. The format changed how players selected moves, targeted opponents, and built teams.
Moves could affect one opponent, both opponents, an ally, or every Pokémon on the field, making positioning and area effects more important than in the one-on-one battles of earlier games. Some trainers and wild encounters used the new format.
Double battles are distinct from later triple and rotation battles. Ruby and Sapphire also introduced Pokémon abilities, natures, Pokémon Contests, Secret Bases, and the Hoenn region, but double battles were the most prominent new change to the battle system.