The square knot is also know as the the reef knot. I have also hear it commonly refereed to as the “joining” or “friendship” knot. It is a simple knot, most commonly used to join two ropes of the same size together. However, The International Guild of Knot Tyers warns that this knot should never be used to join two ropes together. A sheet bend should be used instead. Knotting authority Clifford Ashley claimed that misused square knots has caused more deaths and injuries than all other knots combined.
The square knot is used to tie the two ends of a single line together such that they will secure something, for example a bundle of objects, that is unlikely to move much. It has also been used since ancient times to tie belts and sashes. It is appropriate for tying plastic garbage or trash bags, as the knot forms a handle when tied in two twisted edges of the bag.
The name "reef knot" dates from at least 1794 and originates from its common use to reef sails, that is to tie part of the sail down to decrease its effective surface area in strong winds. To release the knot a sailor could collapse it with a pull of one hand; the sail's weight would make the collapsed knot come apart.
Did you know there are many ways to tie the square knot?
Left over right and right over left
If you tie it wrong this way you will get a granny knot. Which is just two overhands going the same direction, forming what most of us simply call a hard knot. However, in reality this is a lousy knot as it lacks security and jams easily.
Another way to tie the square knot is with a loop. Create a small loop, without crossing the ends, so the long end and the short end are just laying next to each other. Then with the other end of the rope come up out of the loop and go around and behind the loop in the opposite direction from the short end. This way the short end you are tying with ends up on the same side of the loops short end. Then poke the short end your tying with back into the loop and pull all four ends.
If you tie the knot wrong this way you will get whats known as a thieves knot. If you notice the short ends of the rope are on opposite sides of the loop. While a real square knot they would be on the same side. The thieves knot is mostly a trick knot and has no real value as a working knot as it easily slips.
I was once told a story behind the thieves knot and it goes like this. In the old nautical days sailors would tie their bags closed using the thieves knot, if when they returned the knot was tied wrong, they would know someone got into their bag. Of course this screams of myth, after all if everyone did this, everyone would know how to tie it back correctly.
Talking about trick knots here is a cool way to tie a square knot out of a larks head. All you do is run one end of the rope through the loop created by the larks head and pull the larks head over itself.