Pokémon Ruby and Sapphire introduced Double Battles, in which two Pokémon fight on each side.
The format debuted in the third-generation Game Boy Advance games, released in Japan in 2002. Instead of the traditional one-on-one battle, each trainer can have two Pokémon active simultaneously, creating opportunities for moves that affect allies, opponents, or everyone on the field.
Double Battles made positioning, move targeting, and team combinations more important. Pokémon Abilities and moves that interact with partners became especially valuable, and the format later became a major part of competitive Pokémon competitions and official tournament rules.
Double Battles should not be confused with Multi Battles. In a Multi Battle, two trainers are on each side and each controls one active Pokémon; in a standard Double Battle, one trainer controls both Pokémon on their side. Triple and Rotation Battles were introduced in later games.