Well, actually, technically the species of Thamnophis sirtalis is the Common Garter Snake...and any subspecies falling under that species could be considered a Common Garter Snake...according to The Garter Snakes: Evolution and Ecology. (Right? For those of you that have the book...am I reading that correctly?). But like Chantel said, most of the time when people refer to the common garter snake, they are referring to easterns....the subspecies Thamnophis sirtalis sirtalis. And like Chantel said, there are many species of garters and ribbons.