Transfer Pak

The Transfer Pack is an accessory for the Nintendo 64. It plugged into the slot of the N64 Controller and had a slot for Gameboy Color games to be inserted. This device allowed for data to be exchanged between the Gameboy and the N64 games. Notable Mario games that used this device are Mario Golf (Nintendo 64) and Mario Golf (GBC), and Mario Tennis (N64) and Mario Tennis (GBC). This transfer pack allowed for specific characters to be unlocked in either game.

Trivia

 * The Transfer Pack is essentially the predecessor of the Gameboy Advance Cable.