Actually,I have heard the argument that it is the inability to shift that makes it single speed, according to local legend Bill Braum, aka Trainwreck who rides a Schwinn S-10 single speed with an option gear, and a Deore rear derailluer to allow him to be in either gear. The catch is no shifters, hence he has to be off of the bike to switch between the different gears. I am considering a similar set-up for my mountain bike.