The Double Sheetbend is a more secure form of the
Sheetbend.
The thicker line of the two is used to form a bight, and
the thinner line 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 lines varies considerably, or when a
more secure Sheetbend is required.