Aid climbing, climbing using equipment that acts as artificial handhold or footholds, became popular during the period 1920-1960, leading to ascents in the Alps and in Yosemite Valley that were considered impossible without such means. However, climbing techniques, equipment and ethical considerations have evolved steadily. Today, free climbing, climbing on holds made entirely of natural rock, using gear solely for protection and not for upward movement, is the most popular form of the sport. Free climbing has since been divided into several sub-styles of climbing dependent on belay configuration.letic