Which Pokémon video games introduced double battles, where two Pokémon fight for each side?

Pokémon Ruby and Sapphire introduced double battles, where two Pokémon fight for each side.

The feature debuted in the third-generation games for the Game Boy Advance. Instead of sending out one Pokémon at a time, trainers could have two active Pokémon simultaneously, creating new combinations of moves, abilities, and type advantages.

Double battles also changed how players evaluated attacks. Some moves could affect multiple combatants, while others could target an ally, an opponent, or everyone on the field. The format became a lasting part of Pokémon battle design and later appeared in competitive play and other games.

Ruby and Sapphire introduced several other important mechanics, including Pokémon Abilities, Natures, Contests, and 135 new species. They were released in Japan in late 2002 and internationally in 2003, making them the first main-series games of Generation III.