The sheet bend 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 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.
It is a more secure replacement for the reef knot (square knot), especially in its doubled variety.
The Double Sheet Bend. When lines are of unequal diameter or rigidity it is necessary for security to "double" the sheet bend by making an additional round turn below the first and again bringing the working end back under itself. The free ends should end up on the same side of the knot for maximum strength.