The square knot, sometimes incorrectly called a reef knot, is commonly used on boats. It is used to tie a line in a loop around something or to tie together two lines of the same diameter. If tied correctly, it holds well, although it may be difficult to untie if pulled very tight under a strain.
A square knot is tied with two overhand knots. (An overhand knot is the first step in typing shoelaces.) The overhand knots in a square knot must be opposite: right over left, then left over right, as shown in this series of photos.
