Sometimes a difficult knot to accomplish, a blood knot (also referred to a Barrel Knot) is useful when combining sections of a nylon line while also maintaining the lines strength. In fly fishing, this helps to create a section of slowly decreasing length with the fly line hooked to the larger end and hook at the smaller end.
The Barrel Knot is an extremely strong knot when tied properly and should be tied with monofilament close in diameter. Best use is for monofilament should be 10 lbs. and up.