Knot, Reef

The reef knot, or square knot, joins the two ends of the same line and was used to tie off reef points when shortening (reefing) a sail. It is two overhand knots tied in opposite handedness (left over right, then right over left) so it lies flat. It is a binding knot, not a bend: it can slip or capsize under strain on lines of unequal size or material and should never join two ropes that will take a heavy pulling load. Reversed handedness produces the insecure granny knot.