Which Pokémon games introduced two-on-two battles to the main series?

Pokémon Ruby and Sapphire introduced two-on-two battles to the main Pokémon game series.

The games were released in Japan in 2002 for the Game Boy Advance and began Pokémon’s third generation. In a double battle, each side can have two Pokémon active at once, allowing moves and abilities to affect allies as well as opponents.

Ruby and Sapphire also introduced Pokémon abilities, natures, Pokémon Contests, Secret Bases, and the Hoenn region. These systems gave battles and team-building more strategic variety than the one-on-one format used in the original games.

Double battles later became a recurring feature in the series and are especially important in competitive formats. Pokémon Gold and Silver added breeding and held items, but not the two-Pokémon battle format.