Which Pokémon games introduced double battles, with two Pokémon fighting on each side?

Pokémon Ruby and Sapphire introduced double battles, in which two Pokémon can fight for each side.

The games were released for Game Boy Advance in Japan in 2002 and later internationally. Double battles changed the usual one-on-one format by making positioning, move targets, and team combinations important parts of a battle.

Ruby and Sapphire also introduced abilities and natures, two systems that affected how Pokémon performed in battle. Some moves could affect multiple combatants, creating strategies that were not possible in the original games. Later Pokémon titles retained and expanded double battles, but the mechanic first appeared in the third-generation pair.