Update to cilice, with all due respect, the ratings at "fifth class rock" start at 5.0, not 5.1, though anybody would be hard pressed to determine the difference. Where things get "hard" is a personal perception, but the jump from 5.8 to 5.9 is quite large, from 5.9 to 5.10, larger still, and it grows exponentially from there.

There are also additional ratings that sometimes help qualify a climb, like "R" or "X", which have a relationship to what chance you have of actually dieing on the climb (via a Grounder, for example.