Hey, check this out from Wiki. I had looked up the rock climbing system to understand Halhiker's comment to Newmountaineer, and came across this:
The increasing technical difficulty of Class 5 climbs led to the same relative-grading problem that had caused the initial development of the system; as a result, Class 5 was subdivided in the 1950s. Initially it was based on ten climbs of Tahquitz Rock in Idyllwild, California, and ranged from "the Trough" at 5.0, a relatively modest technical climb, to "the Open Book" at 5.9, considered at the time the most difficult unaided climb humanly possible. This system was developed by members of the Rock Climbing Section of the Angeles Chapter of the Sierra Club.[1]
Very interesting - Tahquitz is very well known, I guess.
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