Cube Notation is letters and letter character combinations to describe specific turns on a 5x5 (Professor’s Cube) . The purpose of notation is to easily describe an algorithm or a scramble.
Uppercase letters describe one 90 degree turn of one layer the letter represents.
If the letter is followed by an apostrophe then the turn is anti-clockwise.
- UP -
Capital Letter U is only the top payer

Capital U followed by lowercase w is both the top and inner top layer together

Lowercase u is the inner top layer (not used very often)

The concept above applies to the Right, Left, Back, Front and Down layers.
