The Double Sheetbend is a more secure form of the Sheetbend.
The thicker rope of the two is used to form a bight, and the thinner rope
is passed up through the bight, around the back of the bight, around again
before tucking under itself.
Tip. It is particularly useful when the thickness of the two ropes varies
considerably, or when a more secure Sheetbend is required.