Which Pokémon game generation introduced double battles?

Generation III introduced double battles to the Pokémon video game series.

The mechanic debuted in Pokémon Ruby and Sapphire, released for the Game Boy Advance in Japan in 2002. Instead of sending out one Pokémon at a time, a Trainer could use two simultaneously, creating new possibilities for targeting, support moves, weather effects, and team strategy.

Double battles became a regular part of later main-series games. Certain non-player Trainers also used the format, and some battles required players to coordinate attacks between two Pokémon on the same side.

The term Generation III refers to the Ruby, Sapphire, and later Emerald era, along with the related FireRed and LeafGreen remakes. The games also introduced abilities and natures, but those features are separate innovations from double battles.