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.

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.

YDS class

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.

Decimal system may refer to: