Master System is a console version of the GameGear. It's also a PAL system IIRC and it didn't come out here in the US and we got GameGear instead. I can't think of anything else. D:
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Actually, I think the Master System came out in the US. Think of the Game Gear as the Nomad to the Master System's Genesis.
The Game Gear versions (which are on SGC, SMC+, and SA:DX) have a smaller screen (duh) and apparently have a few more colours (not sure if the colour thing is true).[snapback]7262[/snapback]
The GG had a larger color pool in that the SMS had 256 possible colors, whereas the GG had 4,096. Both could only display 32 at a time, though.