The Yosemite Decimal System (YDS) is a three-part system used for rating the difficulty of walks, hikes, and climbs, primarily used by mountaineers in the United States and Canada. It was first devised by members of the Sierra Club in Southern California in the 1950s as a refinement of earlier systems, particularly those developed in Yosemite Valley, and quickly spread throughout North America. Description The class 5 portion of the class scale is primarily a rock climbing classification system, while classes 1–2 are used mainly in hiking and trail running. Class 3 describes easy and moderate climbing (i.e. scrambling), with varying amounts of exposure (length of a possible fall). Class 4 is an "in-between" rating that describes a very exposed scramble, corresponding roughly to the IFAS classification of PD+. Climbers, specifically those involved with technical class 5 climbing, often abbreviate "class 3" and "class 4" to "3rd" and "4th" respectively. Originally the system was a single-part classification system. "Grade" and "protection" categories were later added to the system. The new categories do not apply to every climb and usage varies widely. "Grade" describes the amount of time needed to complete a climb, and "protection" describes the availability and quality of places on a climbing route where a climber may utilize climbing protection; ratings for these can be subjective. Class 5 is subdivided into parts, currently 5.0 to 5.15. Ratings 5.10 and above are further subdivided, for example, 5.10b or 5.15c. When writing out a full YDS rating, grade is added after classification, and is followed by protection, so a typical YDS route description would be something like "5.10b VI R". Often the grade is omitted, and just the classification and protection ratings used. If the amount of protection available on a route is not concerning, then the protection rating is omitted as well. While the YDS is primarily considered a free climbing system, an aid-climbing designation is sometimes appended, numbered A0 to A5. For example, the North America Wall on El Capitan would be classed "5.8 VI A5" using this mixed system. Guidebooks sometimes 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. Classes The system now divides all hikes and climbs into five classes: The exact definition of the classes is