Adventist Youth Honors Answer Book/Knot/Slipped half hitch
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Use: A slipped half-hitch is a knot used to attach a line to a rod or bar. It does not provide great strength compared to some other knots, but it can be tied relatively quickly and released very easily. These characteristics mean that it is used on square-rigged ships for securing the gaskets that bind stowed sails to the yards.
How to tie:
Begin as for a clove hitch, but instead of passing the end of the line through the loop in the final step, pass a bight instead, leaving the end on the original side. Pulling on this end will release the hitch - if tied in a gasket, this will quickly release the sail.
WARNING: Never use this knot in a critical situation. It comes untied at the slightest provocation.