The Square Knot is the most important decorative knot you will learn to tie. Most Macrame patterns use it in one form or another, so it's essential that you learn to tie it easily in both directions.
The square knot, sometimes incorrectly called a reef knot, is commonly used on boats. It is used to tie a line in a loop around something or to tie together two lines of the same diameter. If tied correctly, it holds well, although it may be difficult to untie if pulled very tight under a strain.