Yosemite Decimal System

"The Yosemite Decimal System (YDS) is a three-part system used for rating the difficulty of walks, hikes, and climbs. It is primarily used by mountaineers in the United States and Canada. The Class 5 portion of the Class scale is primarily a rock climbing classification system, while the Classes 1-3 are used mainly in hiking and trail running. Originally the system was a single-part classification system. In recent years, Grade and Protection categories were added to the system. The new categories do not apply to every climb and usage varies widely. The grades are subjective, reflecting the first ascenscionist's opinion of the climb's difficulty. There are also systematic regional differences in grading within the United States."