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While Nintendo never released an adapter for playing NES games on the SNES (though the instructions included a way to connect both consoles to the same TV by either daisy chaining the RF switches or using AV outputs for one or both systems), the Super Game Boy adapter cartridge allows games designed for Nintendo's portable Game Boy system to be played on the SNES. The Super Game Boy touted several feature enhancements over the Game Boy, including palette substitution, custom screen borders, and (for specially enhanced games) access to the SNES console. Japan also saw the release of the Super Game Boy 2, which added a communication port to enable a second Game Boy to connect for multiplayer games.

What improvements did the Super Game Boy have over the Game Boy?
palette substitution, custom screen borders, and (for specially enhanced games) access to the SNES console