A water knot is used to connect the ends of two pieces of
webbing, or both ends of one piece to form a
loop.
Works only with materials that have a flat cross section, and *not* with conventional ropes or string with a round cross section. Used in rock climbing, mountaineering, and mountain rescue.
To make a water knot in standard one-inch webbing, hold about six inches of the end of a piece so it points away from you. Fold it back over itself once at 45 degrees so it points right, then fold under itself at 180 degrees so it points left. Take the tip and thread it through the loop you've made and gently tighten so you have at least two inches free tail. Now take the end of the other piece and thread it through the first knot *in the opposite direction*. Keep the new end flat against the existing knot as you pass it through the loop, then 45 degrees and under toward the right, 180 degrees over and to the right, and 45 degrees over and away from you.
A proper water knot is perfectly flat and triangular, with no kinks or pinching, and the joined pieces should be straight when under tension, with the two tails off at 45 degrees.
The virtues of the water knot are that it is almost as strong as the webbing itself, and that it will not loosen under alternating tension and slack. The drawback is that they get extremely tight and may require a marlinspike to undo. Climbers often make loops of Nylon webbing with water knots to use as anchors and never undo the knots, simply retiring the loop when the webbing begins to fray.