Blood knot.

About Blood knot

A blood knot is a bend knot most usefully employed for joining sections of monofilament nylon line while maintaining a high portion of the line's inherent strength. Other knots used for this purpose can cause a substantial loss of strength. In fly fishing, this serves to build a leader of gradually decreasing diameter with the castable fly line attached at the large diameter end and the fly or hook at the small diameter end. The principal drawback to the blood knot is the dexterity required to tie it. It is also likely to jam, which is not a concern in fishing line, which is no great loss to cut, but may pose a concern in normal rope. The term "blood knot" can also refer to "a double overhand knot tied in a cat-o'-nine-tails".The barrel knot, called blood knot by Keith Rollo, is the best bend there is for small, stiff or slippery line. The ends may be trimmed short and the knot offers the least resistance possible when drawn through water.