What battle format did Pokémon Ruby and Sapphire introduce to the main series?

Pokémon Ruby and Sapphire introduced Double Battles to the main series.

In a Double Battle, each side uses two Pokémon at the same time. This changed battle strategy because moves could target allies, opponents or multiple combatants, making positioning and move selection more important than in traditional one-on-one fights.

Ruby and Sapphire were released for the Game Boy Advance and began Generation III. Alongside Double Battles, they introduced Pokémon abilities and natures, which added new layers of passive effects and statistical variation.

Multi Battles also appeared alongside Double Battles. They used a similar two-Pokémon field, but involved two trainers on each side. Triple and Rotation Battles arrived in later generations and should not be credited to Ruby and Sapphire.