The sheet bend double is a bend, that is, a knot that joins two ropes together. Doubled, it is effective in binding lines of different diameter or rigidity securely together, although it has a tendency to work loose when not under load.
The double sheet bend is related in structure to the bowline. It is very fast to tie, and along with the bowline and clove hitch is considered so essential it is knot №1 in the Ashley Book of Knots. It is a more secure replacement for the reef knot (square knot), especially in its doubled variety. It's also known as the weaver's knot, becket bend or weaver's hitch.
ABoK (simple) #1, #66, #1431; (double) #488, #1434; (weaver's) #2, #485;
Fold the rope to form a loop with the running end upside.
Thread the 2nd rope through the loop over the 1st running part.
Turn the running end of the 2st rope around both ends of the 1st rope.
Turn over again.
Tuck the 2st rope under itself to finish the knot.
Tighten the knot.
The Sheet Bend Double is complete.