What cartridge feature in Sonic & Knuckles lets players attach other Genesis games?

Sonic & Knuckles uses lock-on technology to let players attach other Genesis games.

The cartridge has a second cartridge slot built into its top. Inserting Sonic the Hedgehog 3 combines both games into the expanded Sonic 3 & Knuckles adventure, with linked progression and additional content. Sonic the Hedgehog 2 can also be attached, making Knuckles playable in Sonic 2 levels.

The feature was created partly because Sonic 3 and Sonic & Knuckles were originally planned as one large game. Cartridge size, manufacturing expense, development time, and Sega's release schedule forced the project to be split. The adapter preserved the intended connection between the two cartridges.

A common mix-up is confusing lock-on technology with Genesis hardware features such as blast processing. Blast processing was a marketing term for the console's perceived speed, while lock-on specifically describes the physical cartridge-linking system. Attaching other games also unlocked the Blue Sphere bonus game, although only selected Sonic cartridges produced major playable expansions.