Where Does F' in Rubik's Cube Group Singmaster Notation Come From?

Question

Basic 90° : F turns the front clockwise
180° : F^2 turns the front clockwise twice
-90° : F' turns the front counter-clockwise

Why are we calling the -90° rotation F' and not -F?

(source: http://en.wikipedia.org/wiki/Rubik%27s_Cube_group)

Answer 1

The group is written multiplicatively, so in standard notation it would be $F^{-1}$, not $-F$. I suspect that $F'$ was used because the original pamphlet — I have one version of it — was typewritten, and a prime was a lot easier to produce than a superscript $-1$. It also makes the notation more compact.