The square knot or even the joining knot is a simple but effective method for securing two equally sized ropes and is among the most commonly known and used knots. Originally used by sailors to reef sails because in heavy seas the knot can be released with one hand. However it is not considered to be safe for long term use and should never be used to connect unequally sized ropes as the knot will eventually slip.
How to tie a reef knot.
Take one end of the rope in each and hand and pass the right over and then under the left, now with the same rope now in the left hand, pass left over right and under. You should now have a knot which is symmetrical and resembles a figure eight. As a scout you may have learned the mantra "right over left and under, left over right and under".
How to untie a reef knot
Grasping either one or both of the top loops of the knot you can easily 'break' any tension that has caused the knot to close, if too much pressure has been applied and the knot will not 'break' easily the application of a marlinspike will create enough space to loosen the knot.
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