The Sheet Bend is a bend knot, which are used for joining two cords, lines, or ropes together. When this particular knot is doubled it can be used to tie ropes of unequal diameter together. Using a single sheet bend to tie ropes of unequal diameter together is unsecure, and most likely will not hold. In addition to unequal diameter ropes, making sure the loose ends are on the same side ensures a safe, secure sheet bend. Like shown in the picture to the right, both loose ends end up on the same side, making it a secure, safe, sheet bend.
The sheet bend is considered so essential that it is knot No1 in The Ashley Book of Knots.
The finished knot: