Reef knot

Canonical Name: reef knot.
Variant Name(s): square knot.

Category: bend.
Origin: Ancient.
Related knot(s): thief knot, granny knot, grief knot.
Releasing: Jamming.
Efficiency: 48%.
Caveat: Generally insecure (contrary to common opinion), very easy to tie, spills easily.
Uses: Used to tie the ends of two lines together such that they will not skip apart (unlike a granny knot). It is also used decoratively.
Comments: The working ends of the two lines of a square knot must be cis (that is, both at the top or both at the bottom); the other lines lead to the full rope. This is important; if the line ends are trans (that is, top on one side and bottom on the other), you produce a thief knot, which may slip out under load. (The "cis" and "trans" terms are derived from terminology used to describe geometric isomerism.)

It is called a reef knot because it was used to "reef" or tie up sails. It is called a square knot from its appearance.

A number of knotting guides have voted this knot "the worst knot", and noted that more deaths and injuries have been caused by it than all other knots combined. Its symmetry and initial feel of security have led to it having a wide and undeserved reputation as a secure knot, which it certainly is not, especially if the ropes are of differing sizes or materials, or if the rope is likely to be bumped.

If you pull on an end, the knot will loosen and invert (turning into a lark's head knot), hence its original purpose: it secured the furled-in sails, and is easily undone one-handedly by a sailor who is holding on with the other hand. The weight of the sail would drag the lark's head undone.

If you want to tie something securely, then investigate the bowline or sheet bend.
Structure:
Tying: To tie a square knot, tie a left-handed overhand knot and then a right-handed overhand knot or vice versa. Do not tie two knots of the same handedness. That makes a granny.

The "boy scout" instructions for this knot are: right-over-left and under; left-over-right and through. (The square knot is the first knot most Boy Scouts learn and has been adopted as a symbol by the movement.)