Double battles debuted in Pokémon Ruby and Sapphire, allowing each side to use two Pokémon at the same time. This format became one of the defining battle innovations of the third generation.
In a double battle, moves can target one opposing Pokémon, an ally, both opponents, or every Pokémon on the field, depending on the move. This changed team-building because players had to consider combinations, positioning, area attacks, and moves that protect or support a partner. The games also introduced multi battles, in which two trainers could cooperate.
Ruby and Sapphire were released for the Game Boy Advance and introduced the Hoenn region. Their other major additions included Pokémon Abilities, Natures, Pokémon Contests, and 135 new species. Double battles are sometimes confused with later triple or rotation battles, but those formats were introduced in subsequent generations.