Sonic & Knuckles / Sonic the Hedgehog 3

Sonic & Knuckles features "lock-on technology" that allows players to open the hatch on the cartridge and insert a second cartridge. When Sonic 3 is inserted, the player can play through both games as one, Sonic 3 & Knuckles. This features several changes to the games, such as slightly altered level layouts, the ability to play through Sonic 3 levels as Knuckles, or Sonic & Knuckles levels as Tails, and the ability to save progress in Sonic & Knuckles levels. Additionally, combining the cartridges is the only way to collect "Super Emeralds", earned by accessing Special Stages in the Sonic & Knuckles levels after collecting all seven Chaos Emeralds from Sonic 3. When all Super Emeralds have been collected, Sonic, Knuckles, and Tails can transform into Hyper Sonic, Hyper Knuckles, and Super Tails respectively, each with unique abilities.