Wikipedia has a bit of info. http://en.wikipedia.org/wiki/Yosemite_Decimal_System#YDS_class http://en.wikipedia.org/wiki/Yosemite_Decimal_System#YDS_cla... Th

Wikipedia has a bit of info. http://en.wikipedia.org/wiki/Yosemite_Decimal_System#YDS_class http://en.wikipedia.org/wiki/Yosemite_Decimal_System#YDS_cla... The difficulty is rated as one pitch relative to another and certain pitches as benchmarks for a certain difficulty. e.g. pitch X is 5.13a and pitch Y is somewhat harder so it's 5.13c while Z is quite a bit harder making it a 5.14a, etc.