The letters M, S and E are used to denote the turning of a middle layer. M represents turning the layer between the R and L faces 1 quarter turn top to bottom. S represents turning the layer between the F and B faces 1 quarter turn clockwise, as seen from the front. E represents turning the layer between the U and D faces 1 quarter turn clockwise left to right. As with regular turns, a 2 signifies a double turn and a prime (') indicates a quarter turn anticlockwise.