The History of the Yosemite Decimal System

The Yosemite Decimal System, or YDS, is a three-part system for rating the difficulty of walks, hikes, and climbs, used for mountaineering primarily in the United States and Canada. The Class 5 portion of its Class scale is the primary rock climbing classification system used in those locales.
Class 1: Walking with a low chance of injury.
Class 2: Simple scrambling, with the possibility of occasional use of the hands. Little potential danger is encountered.
Class 3: Scrambling with increased exposure. A rope can be carried but is usually not required. Falls are not always fatal.
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 involving rope, belaying, and other protection hardware for safety. Un-roped falls can result in severe injury or death.

Increased standards and improved equipment meant that class 5.9 climbs in the 1960s became only of moderate difficulty for some. Rather than reclassify all climbs each time standards improved, additional classes were added. It soon became apparent that an open-ended system was needed and further classes of 5.11, 5.12, etc. were added. 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 suffixes from "a" to "d".

R: Runout, some protection placements may be very far apart (possibility of broken bones, even when properly protected).
X: No protection, extremely dangerous (possibility of death even when properly protected).