A turn and its undo

Cubers name the six faces by where they sit: U for up, D for down, R right, L left, F front, B back. "R" means turn the right face a quarter turn clockwise, as if you were looking straight at that face. "R′" (said "R prime") is the same face turned back the other way.

Every move has an undo. To undo a sequence, undo the last move first and work backwards: the undo of "R U" is "U′ R′". It is like taking off shoes and socks in the opposite order you put them on.