Dynamic rope

A dynamic rope is a specially constructed, somewhat elastic rope used primarily in rock climbing, ice climbing, and mountaineering. This 'stretch' is what makes it 'dynamic', in contrast to a static rope that has very low elongation under load. Greater stretch allows a dynamic rope to absorb the energy of a sudden load such as from a fall more slowly, reducing the peak force and therefore the chance of catastrophic failure. Kernmantle ropes are the most common type of dynamic rope, and nylon has replaced all natural materials such as hemp since 1945 for durability and strength.

Dynamic climbing ropes are classified into three categories: Single ropes, twin ropes, and half ropes (also referred to as 'double ropes').

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.

Dynamic ropes used for rock climbing come in a variety of lengths and diameters, with the most common lengths being 50, 60, and 70 meters (approximately 165, 200, and 230 feet, respectively). 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 a multi-pitch climbing attempt due to being only slightly longer than a standard rope length.

Rope diameters are generally between 8.3mm and 11.5mm, with the different diameters used for slightly different purposes. Sport and multi-pitch trad climbers often value thinner ropes because they are lighter, and have less rope drag. Thinner ropes also run more smoothly through belay devices, especially assisted braking devices or 'tube-style' devices operated in 'guide mode', which can be tedious to pull thicker or stiffer rope through. Lighter, thinner ropes, however, have less strength than a thicker rope and will sustain fewer hard falls. Note that some belay devices are better suited for different rope diameters. This is particularly relevant with assisted braking devices, such as the Petzl Grigri (which, for example, works best with a 9.4-10.3 mm thick line) or the Faders SUM. Users must make sure to read the instructions for the device carefully to ensure safety and recognize any limitations to rope diameter.

...
Wikipedia