The reef knot or square knot is a common and simple binding knot.
Used to tie two ends of a single line together such that they will secure something that is unlikely to move much. It lies flat when tied with cloth, and has been used for bandages for millennia. With both ends tucked (slipped) it becomes a good way to tie shoelaces, whilst the non-slipped version is useful for shoelaces that are excessively short. It is also used decoratively and to tie the Obi (or belt) of a Karate Gi. Finally, it is quite handy for tying plastic garbage or trash bags, as the knot forms a handle when tied in two twisted "ears".
This knot's name originates from its use to "reef" sails (tie part 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.
The reef knot is one of the key knots of macrame textiles.
Tying a reef knot
To tie a reef knot, tie a left-handed overhand knot and then a right-handed overhand knot or vice versa. (Two consecutive overhands of the same handedness will make a granny knot.) A common mnemonic for this procedure is "right over left, left over right".
The working ends of the reef knot must be cis (that is, both at the top or both at the bottom); the other lines lead to the full rope. Otherwise, a thief knot results. (The "cis" and "trans" terms are derived from terminology used to describe geometric isomerism.)