Yosemite Decimal System

[1] It was first devised by members of the Sierra Club in Southern California in the 1950s as a refinement of earlier systems from the 1930s,[2] and quickly spread throughout North America.

[6] Climbers use class "5" as a prefix, which is then followed by a decimal point and a number that starts at 1 and counts up with increasing difficulty (e.g. 5.4, 5.5, 5.6, etc.).

[1][4] The American YDS system is an open-ended scale, with the current hardest climb being 5.15d, established by Silence in 2017.

[3] Royal Robbins, Don Wilson, and Chuck Wilts came up with a decimal subdivision of the fifth class consisting of 5.0, 5.1, and so on up through 5.9.

This system was implemented in the early 1950s, with new routes and ratings at Tahquitz being described in mimeographed newsletters of the Rock Climbing Section of the Angeles Chapter of the Sierra Club.