The reef knot (landlubbers call it the square knot) often is used to tie the sail cover on to the mainsail or mizzen aboard a sailing ship. This knot’s name originates from its use to “reef” sails (tie part of the sail down to decrease effective surface area), where its easy-spilling behavior was very handy. A sailor could collapse it with a pull of one hand; the sail’s weight would make the collapsed knot come apart.

Step 1: Take two ropes.

Step 2: Take the two ends, cross them over and tie the first half knot (half hitch).

Step 3: Maintain the same rope on top as you cross them over a second time to tie the second half hitch.

Step 4: Pull the ends evenly to form a symmetrical reef knot.

Captain Gerry demonstrates how to tie a reef knot in this video: