The letters X, Y and Z are used to signify cube rotations. X signifies the rotation of the cube forwards 90 degrees. Y signifies the rotation of the cube to the left 90 degrees. Z signifies the rotation of the cube clockwise 90 degrees. These cube rotations are used in algorithms to make the algorithms smoother and faster. As with regular turns, a 2 signifies a half rotation and a prime (') indicates a quarter rotation in the opposite direction. Note that these letters are usually lowercase instead.