The reef knot, is a simple binding knot used to secure a rope or line around something. It is also known as the square knot or Hercules knot.
The common rhyme to tie this knot is "left over right and under, right over left and under".
The reef knot figures prominently in Scouting worldwide emblems. It is included in the copyrighted WOSM international membership badge.
The reef knot or square knot consists of two overhand knots, one left and one right, one being tied on top of the other, and either being tied first...The reef knot is unique in that it may be tied and tightened with both ends. It is universally used for parcels, rolls and bundles. At sea it is always employed in reefing and furling sails and stopping clothes for drying. But under no circumstances should it ever be tied as a bend, for if tied with two ends of unequal size, or if one end is stiffer or smoother than the other, the knot is almost bound to spill. Except for its true purpose of binding it is a knot to be shunned.The Ashley Book of Knots
The reef knot is used to tie the two ends of a single line together to bind something, for example a bundle of objects. In addition to being used by sailors for reefing and furling sails, it is also one of the key macrame knots.
In pioneering it is used as a binding knot to finish off Japanese square lashing and the sailmaker's whipping. However, it is an insecure knot, unstable when jiggled, and is not suitable for supporting weight.
With both ends tucked (slipped) it becomes a good way to tie shoelaces, whilst the non-slipped version is useful for excessively short shoelaces. It is appropriate for tying plastic rubbish bags, as the knot forms a handle when tied in two twisted edges of the bag.
The reef knot's familiarity, ease of tying, and visually appealing symmetry is its weakness.
The International Guild of Knot Tyers warns that this knot should never be used to bend two ropes together.
Knotting authority Clifford Ashley claims that misused reef knots have caused more deaths and injuries than all other knots combined.