The Blood Knot is a favorite knot for fly fisherman. It is primarily used to join two lines of similar size, and is one of the best knots for this purpose. The strength of the knot depends on making at least five, and up to seven, turns on each side of the center.
When lubricated and pulled tight, the knot changes its structure. Pulling on each line forces the wrapped turns to redistribute the twists so that the inner strand becomes an outer wrap.