Square Knot
The square knot is a simple joining knot used to tie two ropes together. 
Square knot - comes from the knot's shape
Joining knot - the knot can be used for joining two ropes.  It is also the first knot required to be tied when joining a Boy Scout troop.
Reef knot - a nautical name because sailors used this knot to "reef" or roll up a ship's sails.
The square knot is useful in many everyday situations that require two ropes to be joined together.  For example, tying a rope around a bundle of newspapers or branches.  The knot used to tie your shoelaces is a slight modification of the square knot.  Do not use the square knot in any situation in which safety is a consideration.  The square knot is unreliable when a heavy strain is placed on it.
How to Tie
1.  Situate your rope so that you have one end of the rope in your right hand.  The other end goes in your left hand.
2.  Place the right end over the left end, then twist the top rope underneath.
3.  Now place the (new) left end over the (new) right end, and twist the (new) top rope underneath.
4.  Tighten by pulling both ends simultaneously with even force.

Stephen C. Dibler, Last update: 12/19/2002