Which Pokémon games introduced Double Battles, in which each trainer uses two Pokémon at once?

Pokémon Ruby and Sapphire introduced Double Battles, in which each trainer can use two Pokémon at once.

Released for the Game Boy Advance, Ruby and Sapphire expanded the battle system beyond the traditional one-on-one format. Double Battles placed two Pokémon from each side on the field, creating new tactical possibilities involving targeting, support moves, abilities, and attacks that could affect multiple Pokémon.

The mechanic became a lasting part of the main series and later influenced official competitive Pokémon formats. It also shaped several story encounters in the Hoenn region, including battles against the twin Gym Leaders Tate and Liza, whose team-based battle is designed around the new format.

Double Battles are distinct from Multi Battles. In a Double Battle, one trainer generally controls both Pokémon on their side. Multi Battles involve multiple trainers cooperating, although both formats can place four Pokémon on the battlefield. Ruby and Sapphire also introduced Pokémon Abilities, Natures, and Pokémon Contests.