Although there are different designs, most fixie bicycles are among the simplest two-wheelers on the road and trail. They're usually comprised of a frame, wheels, bars, stem, seat, seatpost, crankset, pedals and chain. There are no derailleurs and often no visible brakes (you slow/stop by holding back on the pedals). These elegant bikes are called "fixies," because the distinguishing characteristic is having only one fixed gear (it's attached to the rear wheel in such a way that you can't coast, the pedals always go around when the bike is rolling).