Clean climbing is a rock climbing term that describes techniques and equipment which climbers use in order to avoid damage to the rock. These techniques date at least in part from the 1920s and earlier in England, but the term itself may have emerged in about 1970 during the widespread and rapid adoption in the United States and Canada of nuts (also called chocks), and the very similar but often larger hexes, in preference to pitons, which damage rock and are more difficult and time-consuming to install. Pitons were thus eliminated in North America as a primary means of climbing protection in a period of less than three years.