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.

While primarily considered a free climbing system, an aid climbing designation is sometimes appended. For example, The North America Wall on El Capitan would be classed "VI, 5.8, A5" using this mixed system.

The system was initially developed as the Sierra Club grading system in the 1930s to classify hikes and climbs in the Sierra Nevada. Previously, these were described relative to others. For example, Z is harder than X but easier than Y. This primitive system was difficult to learn for those who did not yet have experience of X or Y. The club adapted a numerical system of classification that was easy to learn and which seemed practical in its application.

Guidebooks often append some number of stars to the YDS rating, to indicate a climb's overall "quality" (how "fun" or "worthwhile" the climb is). This "star ranking" is unrelated to the YDS system, and varies from guidebook to guidebook.

The system now divides all hikes and climbs into five classes: The exact definition of the classes is somewhat controversial, and updated versions of these classifications have been proposed.

Information about the difficulty of a summit block is sometimes added. For example, a rating of 3s4 means that most of the climb is class 3 but the summit block is class 4.

The original intention was that the classes would be subdivided decimally, so that a class 4.5 route would be a climb halfway between 4 and 5. 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.

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