The impossible wheel, also known as the BC wheel (after the comic strip B.C.), represents the most basic type of unicycle, consisting of a single, spoked wheel with pegs extending from the axle. The rider mounts the wheel by jumping onto it while in motion or by putting one foot onto one plate or peg and pushing along like a skateboard.
This contraption has no pedals, so when the wheel's inertia runs out due to gravity and friction, the ride ends. Newer BC wheels use plates instead of pegs which drop below the axle. This makes the BC wheel easier to ride since the center of gravity is lower.
The unicycling community favors the term BC wheel because the device is not actually impossible to ride; proficiency can be gained with a few hours of practice. Some people are able to do tricks on BC wheels such as hopping, drops, grinds, and spins.