square knot

1. Hold one end of each of the ropes in each hand.
2. Place the right end over the left one , loop it under and then back up (like the beginning of tying your shoe).
3. Take the new left side (which was the right one before step 2) and repeat step 2 placing the left over the right and looping it around it.
4. Grab each side of the knot in each hand an pull until tight.

The square knot is somewhat over-rated; for joining two lines of similar size together, you might be better off with a sheet bend. I also like the Carrick bend, but that's a much more obscure knot.

The square knot is the same knot as the reef knot. You can think of a square knot as being made up of two consecutive overhand knots, executed as mirror images. If you mistie a square knot (using identical instead of mirror-image overhand knots), you might end up with a granny knot. Finally, even if you tie it correctly, make sure you tighten both ropes equally; if you over-tighten one, the other rope will pop out and into a larks head hitch.