Mountain climbs can be rated using different methods. TourOfCalifornia.org follows the Yosemite Decimal System (YDS), a method popular in North America.
The original Yosemite Class System was created by the Sierra Club in the 1930s to classify the difficulty of hikes and climbs in the Sierra Nevada. In the 1950s a decimal extension was added to further classify the difficulty of rock climbs at Tahquitz and Suicide Rock in Southern California. The original YDS ranged from 5.0 to 5.9, but as climbers and gear got better the scale was opened. 5.10 and harder routes are subdivided into letters a through d, currently up to 5.15c. A route is rated based on its most difficult pitch, not on its overall difficulty.
Mount Whitney, East Face
Class 1: Trail hiking
Class 2: Off-trail hiking
Class 3: Scrambling (downclimb facing outward)
Class 4: Non-technical climbing (downclimb facing inward, often roped)
Class 5: Technical climbing (5.0-5.9 and 5.10a-5.15d, easy to extremely difficult)
By: Bernd Straehle
See Also: Climbing & Mountaineering