A dynamic rope is a specially constructed, somewhat elastic rope used primarily in rock climbing, ice climbing, and mountaineering. This elasticity, or stretch, is the property that makes the rope dynamic—in contrast to a static rope that has only slight elongation under load. Greater elasticity allows a dynamic rope to more slowly absorb the energy of a sudden load, such from arresting a climber's fall, by reducing the peak force on the rope and thus the probability of the rope's catastrophic failure. A kernmantle rope is the most common type of dynamic rope now used. Since 1945, nylon has, because of its superior durability and strength, replaced all natural materials in climbing rope. Rope types Dynamic climbing ropes are classified into three categories: Single ropes, twin ropes, and half ropes (also referred to as 'double ropes'). Single ropes are designed to be used alone, and are by far the most common, and used for top-roping, sport climbing, and trad climbing. Twin and half ropes are used for lead climbing, and are designed to be used as a pair - they are not strong enough to be safely used on just a single strand, and they are tested to different standards as a single rope. Twin ropes are used by treating the pair of ropes as a single rope, clipping both ropes through the same carabiner at each piece of protection. Half ropes are also used as a pair, but only one rope is clipped through each piece of protection- the climber alternates which rope is clipped through each piece. On wandering routes where protection is placed far apart on either side, half ropes can significantly reduce rope drag. Both twin and half ropes have the advantage of redundancy, as well as allowing a rappel along the full length of the climbing rope (by tying both ropes together), so that climbers can descend from a long multipitch route with fewer rappels than with a single rope. Some ropes are 'triple rated', meeting the standards for all three rope types, so they can be used in each configuration. Length and diameters Dynamic ropes used for rock climbing come in a variety of lengths and diameters, with the most common lengths being , , . Lengths will vary depending on rope maintenance and age, and there are even ropes as long as 80 meters for specialized ascents on routes that would normally require