Reef knot

Reef knot
Names	Reef knot, Square knot, Hercules knot, Double knot, brotherhood knot
Category	Binding
Origin	Ancient
Related	Thief knot, Granny knot, Grief knot, Surgeon's knot, Shoelace knot
Releasing	Jamming
Typical use	Joining two ends of a single line to bind around an object.
Caveat	Not secure as a bend unless secured by additional knots(ex: overhand). Spills easily if one of the free ends is pulled outward. Does not hold well if the two lines are not the same thickness.
ABoK	#74, #75, #460, #1204, #1402, #2096, #2573, #2574, #2577, #2580
Instructions	[1]

The reef knot, or square knot, is an ancient and simple binding knot used to secure a rope or line around an object. It is sometimes also referred to as a Hercules knot or Heracles knot. The knot is formed by tying a left-handed overhand knot between two ends, instead of around one end, and then a right-handed overhand knot via the same procedure, or vice versa. A common mnemonic for this procedure is "right over left; left over right", which is often appended with the rhyming suffix "... makes a knot both tidy and tight". Two consecutive overhands tied as described above of the same handedness will make a granny knot. The working ends of the reef knot must emerge both at the top or both at the bottom, otherwise a thief knot results.

The reef knot or square knot consists of two half 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^[1]

The reef knot is not recommended for tying two ropes together, because of the potential instability of the knot when not stabilized; something that has resulted in many deaths (see Misuse as a bend).