The Yosemite Decimal System is a numerical system for rating the difficulty of walks, hikes, and climbs, primarily used for mountaineering in the United States. The rock climbing (5.x) portion of the scale is the primary climb grading system used in the US.

The scale was initially developed as the Sierra Club grading system in the 1930s to rate hikes and climbs in the Sierra Nevada range. Previously, hikes and climbs were described relative to others ("harder than X, but easier than Y"), but this made it difficult for those who hadn't done the other hikes or climbs to understand the comparison, so the numerical grading system was an attempt to codify this into a single scale.

Currently, according to the climbing textbook Mountaineering: The Freedom of the Hills, the system divides all hikes and climbs into five classes:

• Class 1: Hiking.
• Class 2: Simple scrambling, with possible occasional use of the hands.
• Class 3: Scrambling, a rope can be carried but is usually not required.
• Class 4: Simple climbing, with exposure. A rope is often used. Natural protection can be easily found. Falls may well be fatal.
• Class 5: Technical free climbing. Climbing involves rope, belaying, and other protection hardware for safety.

The original Sierra Club grading system also had a Class 6, for artificial, or aid climbing. This sort of climbing uses ropes and other equipment for progress (e.g. climbing a rope up a sheer face with no holds). Class 6 is no longer widely used, however, and artificial climbs today are graded on a separate scale from A0 through A5.

Note that the exact definition of the classes is somewhat controversial [1].

The increasing technical difficulty of Class 5 climbs led to the same "relative grading" problem that had caused the initial development of the system, so that class was subdivided in the 1950s. Initially it was based on ten climbs in Taquitz, California , and ran 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. However, advances in techniques and equipment have since led to harder climbs being completed. The first such climb was given the rating 5.10; the second the rating 5.11. It was later determined that the 5.11 climb was much harder than 5.10, leaving many climbs of varying difficulty bunched up at 5.10. To solve this, the scale has been further subdivided above the 5.9 mark with a-d suffixes. It is now an open-ended scale, with 5.15a the hardest climb having been completed (as of October 2003).

Reference and Source

• Mountaineering: The Freedom of the Hills, 6th Edition, The Mountaineers, Seattle, Washington, ISBN 0898864267