"An abseil/rappel is a controlled descent off a vertical drop, such as a rock face, using a rope. "Abseil" is the normal term in the UK, whereas "Rappel" is used in the US.
This technique is used by climbers, mountaineers, cavers ... to descend cliffs or slopes when they are too steep and/or dangerous to descend without protection... To descend safely, abseilers use a variety of techniques to increase the friction on the rope to the point where it can be controlled comfortably... Practitioners choose a technique based on speed, safety, weight and other circumstantial concerns."
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