Reef Knot
The reef knot, also known as a square knot, is a very commonly used knot. It's general purpose is to attach the ends of one line to make a loop, or to join two lines of the same diameter. If the lines are of different diameter, a sheet bend should be used instead. Reef knots are used on sail ties and clew tie-downs. You will know that the knot is correct if you can squeeze the ends together and create two interlocking loops that slide easily (see the end of the demonstration video).
How to tie it
Step 1: With one end in each 
hand, cross the ends, with the left side (red) crossing on top of the right.
Step 2: Put the left end (that 
is on top) under and through the loop. This is the same as the first step to tying your shoes.
Step 3: Cross the new right end 
(old left end, red in the image) on top of the left.
Step 4: Put the right end under 
and through the space between the first half-knot and the new half-knot.
Step 5: Pull the knot tight.
Common Errors
The one problem that many people encounter when learning this knot is mixing up the new right and left in step 3. When this happens, the old right ends up cross on top, instead of the new right. If this sounds a bit confusing, watch the demonstration video and remember "left over right and under; right over left and under".
