More details about the The Water Knot
Structure: The Water Knot (ABOK # 296, p 50) is essentially an overhand knot (picture on left) with the second strap (or rope) passed along the knot in the reverse direction. The knot should be arranged neatly and pulled tight. Several inches of the straps should be left over and for safety some authorities recommend tying an overhand knot in the ends. In webbing straps it makes a very satisfactory and safe join.
Uses: In climbing it is used to join two pieces of webbing strapping.
Caution: The Water Knot is reported to slip over time especially with repeated loads - see Study by Tom Moyer. For this reason it is recommended to leave long tails and to inspect the Water Knot to check it has not slipped significantly.