5.11 started in Modesto, California as a clothing line created by rock climber Royal Robbins. ... The name "5.11" comes from the highest rock climbing difficulty level that was listed in the Yosemite Decimal System, which was developed by Robbins in the 1950s.
https://www.climbing.com/.../royal-robbins-a-timeline.../
tom_frost_-_tenement_flat_-_1964.jpg