From Speedsolving.com Wiki
In 2001 he extended the standard cubing notation by adding x x' y y' z z'. He stated:
There are many websites that give the solutions for all the various cube positions you can run into when solving the cube using the 'Fridrich method', but most of them just give a set of moves. It takes a lot of time to adjust all the moves so that they can be executed at high speed. I adjusted many algorithms and added cube rotations to the standard notation to make it easy to see how the move can be performed.
- Peter Jansen's page on Speedcubing.com