Fly Fishing Knots: Blood Knot Video
The Blood Knot, also known as a Barrel Knot, is the best knot for connecting monofilament where sections do not vary more than .002" in diameter. It's a very clean, smooth knot.
See detailed instructions below the video:
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How to Tie the Blood Knot
2. Cradling the standing end of each strand in the last two fingers of each hand, start by winding the tag end of one strand around the standing part of the other strand, working away from the X. Use your thumb and forefinger to make these turns. Keep the loop formed at the X open by pinching it with the thumb and forefinger of the hand you are not using to wind.
3. Use 3 turns on each side with monofilament bigger than 0.17" in diameter. With material from 0.15" to 0.10", use 5 turns; and for material 0.009" and smaller, use 7 turns. Pass the end you have just been winding through the loop at the X.
4. Pinch the loop with the thumb and forefinger you have just been using to wind. Repeat the winding process on the other side with the same number of turns. Pass the second tag end back through the same loop as the first tag end in the opposite direction.
5. Make sure that the tag ends stick out from the loop enough so that they don't slip out when you tighten the knot. Don't pull on these tag ends when tightening the knot. Moisten the knot and pull on both standing parts quickly. The barrels on both sides of the knot should form neat coils; if not, cut the strands and start over. Trim the tag ends close to the knot.