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Yosemite Decimal System

A scale for rating the difficulty of outings ranging from day hikes to extreme mountaineering. It was first devised by the Sierra Club in the 1930s, when it ran from 1 to 6, but today it runs from 1 to 5. The 5 category is the only one subdivided decimally, and it is open-ended: new numbers are added as climbers accomplish ever more difficult climbs. In principal, the rating of a climb is based on its most difficult portion, called the “crux.”

1: Hiking on a clear, well-maintained trail. Could do it on a bike.

2: Hiking that requires route-finding skills, through thick brush, over fallen trees or rough talus.

3: So rough you must use your hands to keep from falling.

4: Steep terrain, rope often required.

5: Technical climbing.

resources

Don Grayson, Kurt Hanson, editors.
Mountaineering, The Freedom of the Hills. (6th ed.)
Mountaineering Books, 1997.

A good discussion of the scale can be found at climber.org