Square Knot
The square knot is the only knot that you are REQUIRED to know to achieve the "Scout" rank - the reason for this is its simplicity of tying and its usefulness. It is often called "The First Aid Knot" because so many bandages and splints are tied off with square knots. If you forget it, you will definitely remember it while working on First Aid Merit Badge. Outside of the United States, this knot is referred to as "The Reef Knot" as Europe had another knot that was known already as the square knot prior to the American naming of this knot reaching their shores.
Some uses of the square knot:
Tying two ropes together of similar size and texture
Tying trash bags (still the undisputed king)
Tying shoelaces when not much lace is available
Tying off bandages requiring snugness
Tying thread after sewing or surgery
Tying off splints
Binding (as in bales of straw)
When to NOT use the square knot:
If you read knot-tying books you will find ENDLESS hatred and stories of the foolishness of using this knot - primarily because people have used it in a way that is improper. The square knot is NOT a climbing knot, nor should it ever be used to suspend large amounts of weight. It is likely to break if one end is pulled outward causing a release of tension. It is also notoriously unstable when used to join ropes of differing widths or textures. In fact, the name "reef knot" comes from sailors using it to tie their reef sails; this was useful as they could collapse the knot just by pulling hard on one end. Despite the many horror stories, it's still a darned useful and simple knot when used properly. Complaining about the square knot being bad for something it's not designed for is a lot like complaining that your dog is really bad at purring.
How To Tie The Square Knot:
We often teach this knot with the phrase "Right over left, and left over right" referring to the right end of the rope crossing over the left end, looping, and becoming the left end. This same end is then taken over the (now) right end. In simple steps:
1.) Place one end of each rope in your hands.
2.) Tie an overhand knot, passing the rope in your right hand over and then under the rope in your left hand. At this time, the rope ends have switched hands such that the rope that began in your right hand is in your left and vice versa.
3.) Next, tie another overhand knot, this time passing the rope in your left hand over the rope in your right hand.
4.) Tighten the knot by pulling on your two working ends.
For an animation of the square knot being tied, click HERE or HERE.