Which accessory let Pokémon Stadium connect with Pokémon Red, Blue, and Yellow?

The Transfer Pak let Pokémon Stadium connect with Pokémon Red, Blue, and Yellow.

The accessory plugged into the back of a Nintendo 64 controller and held a Game Boy cartridge. In Pokémon Stadium, it allowed players to use Pokémon from their handheld games in three-dimensional battles, as well as view, organize, store, and trade Pokémon.

This connection gave Stadium a major advantage over a standalone battle game. Players could train Pokémon in the Game Boy titles and then bring them into the Nintendo 64 game, rather than relying only on teams supplied by Stadium itself.

The Transfer Pak is sometimes confused with the Game Link Cable, which connected two Game Boy systems directly. It is also different from the Nintendo 64 Controller Pak, a separate memory accessory. Pokémon Stadium was released in Japan in 1999 and reached Western regions in 2000.