any of numerous knots used in tree ~I climbing or rigging that may alternately slide along and then grip a rope.
friction hitch (Wikipedia)
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A friction hitch is a kind of knot used to attach one rope to another in a way that is easily adjusted. These knots are commonly used in climbing as part of single-rope technique, doubled-rope technique and as "ratchets" to capture progress on a moving rope, most typically in a mechanical advantage system such as a Z-drag. These hitches are a simple and cheap alternative to mechanical ascenders.