Tricycles.....got it. I was right, you can't use them interchangeably!
Well... a lot of commercial single speed bikes have a fixed/free hub. One side of the hub has a fixed cog, so if that's hooked to the chain the bike is a fixie. The other side has a freewheel, so if *that* cog is hooked into the chain, the bike is a singlespeed. So these bikes can sort of be both at once, since you can use them either way. They also come with brakes, which some fixed gear riders think is heresy. I like being able to stop, so I like brakes.
around here if you want your fixie to be street legal you have to have at very least a front brake on it. (a single speed must have brakes otherwise - no way to stop!)
What Torillin is referring to is called a flip-flop hub, fixed on one side free on the other, so you can choose depending on your situation. You can also set them up with a different gear on either side.
"Sharing the road means getting along, not getting ahead" - 1994 Washington State Driver's Guide
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Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett