The blood knot
The blood knot joins two lines of similar diameter into a neat, slim, in-line join. Each line wraps round the other five to seven times and tucks back through the middle, and the two coils pull together into a tidy barrel. It is the classic knot for building a tapered mono leader, where a flat, straight join matters.
Lay the two line ends alongside each other so they overlap by a good length, pointing in opposite directions. Hold them crossed at the middle where the join will form, leaving each tag end free to wrap.
Take one tag end and wrap it round the other line five to seven turns, working away from the crossing point. Then bring that tag back and pass it through the gap at the middle, where the two lines first crossed. Pinch it there so it holds.
Take the other tag end and wrap it round the first line the same number of turns, five to seven, but the opposite way. Bring it back and pass it through the same central gap, from the opposite side to the first tag. The two tags now point out of the middle in opposite directions.
Wet the knot. Pull both standing lines slowly apart in opposite directions. The two sets of wraps gather and roll toward each other, closing the central gap and forming a neat barrel. Pull evenly so both coils tighten together rather than one over-running the other.
Keep drawing the standing lines apart until the barrel is firm and the coils sit snug. Check the join is neat and in line with both lines, with no loops sticking out, then trim the two tag ends close.
Five steps. Each one is a panel in the diagram above. Wet it before you pull it tight.
What it ties
Two lines of similar diameter, joined end to end into a neat, slim barrel. It is the classic knot for building a tapered leader, where you join lengths of mono or fluorocarbon that step down in thickness toward the fly. Each line wraps round the other and tucks back through the same gap in the middle, so the finished join sits flat and straight, in line with both lines rather than off to one side. That tidy, in-line shape is the reason fly anglers use it for leaders. It is at its best in mono and fluorocarbon of similar diameter; it is fiddlier and less reliable when the two lines are very different thicknesses. For a faster join, especially with cold hands, the surgeon's knot does the same job; the blood knot is the neater one.
When to use it
Use the blood knot when you want a neat, flat join between two lines of similar diameter and you have a little time to tie it, which is when you are building or tidying a tapered leader. It sits slimmer and straighter than a surgeon's knot, so it passes through the rod rings cleanly and does not catch weed or hinge the leader. Keep five to seven wraps on each side, with the same count both sides so the coils gather evenly. The thing to watch is that both tag ends tuck back through the same central gap, from opposite sides; get that wrong and it will not draw down. If your fingers are cold or the light is going, a surgeon's knot is the faster choice for the same job.
Strength and tips
Tied well the blood knot is strong and very neat, and its slim, in-line shape is the whole reason to choose it for a leader. Two things make it. First, keep five to seven wraps on each side, and use the same count both sides so the two coils gather evenly when you pull; too few wraps and it slips, uneven counts and it draws down crooked. Second, both tag ends must tuck back through the same central gap, from opposite sides; that opposed tuck is what locks the two coils together. Wet it before you draw it down, and pull both standing lines slowly and evenly so the barrel forms square. It is at its best on two lines of similar diameter; for very unequal lines it loses grip, and there a surgeon's knot copes better. The surgeon's is also the quicker tie if you are cold.
1Wet it
Wet every knot before you pull it tight. A dry knot drags against itself as it closes and the friction heat weakens the line.
2Seat it slowly
Draw it down slowly and evenly, then trim the tag end close, leaving a stub of a millimetre or two so it cannot slip back through.
3Test it
Pull the finished knot firmly against your hand or the rod before you fish it. Better it fails now than on the take.
Rigs that use it
The blood knot builds the tapered mono leaders on the fly rigs. It joins the stepped-down sections of leader on the dry-fly rig, the nymph rig and the streamer rig, the neat, flat join you want where the leader has to turn over cleanly and pass through the rings. It is the tidy leader-building knot across all three.
Blood knot questions
Joining two lines of similar diameter into a neat, slim, in-line join. It is the classic knot for building a tapered mono or fluorocarbon leader, where the sections step down toward the fly. The flat, straight shape passes through the rod rings cleanly and turns the leader over well.
Five to seven on each side, and the same count both sides. Too few and the knot can slip; uneven counts and it draws down crooked. Both tag ends then tuck back through the same central gap from opposite sides. Wet it and pull both standing lines apart slowly to gather the coils.
Use a blood knot for a neat, flat, in-line join when you have a little time, which suits building a leader. Use a surgeon's knot when you want speed, especially with cold hands or fading light. Both join two lines of similar diameter; the blood knot is sleeker, the surgeon's is faster.