This very useful knot comes from the mountaineering world but has many uses for sailors as well. It’s a non-jamming friction hitch. It’s very simple to tie but requires a purpose made loop from a smaller diameter rope than the one you are tying onto. Once the knot is tied you can slide it up and down the main rope with your hand over the knot but as soon as load is applied the knot locks up. It is very useful for various “tighten and lock” applications. It also is very often used as a fall arrest knot and can be employed when climbing the mast. In fact in a pinch when no climbing gear is available you can use just two prusik knots with some long loops to climb the rig.
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