The blood knot is the classic way to join two lines of similar diameter into a strong, streamlined connection. It is the knot fly anglers use to build and repair tapered leaders, stepping down from a thick butt to a fine tippet, and it splices mono cleanly anywhere you need a slim, inline join.
When and why to use it
Reach for the blood knot when you are joining lines of roughly the same diameter: building a hand-tied tapered leader, adding a section of tippet, or splicing mono. It sits slim and straight, so it passes through the guides well. For lines of very different diameters, or braid to leader, use a double uni or FG instead.
How it works
You overlap the two lines, wrap one tag around the other standing line several times and bring it back to the middle, then repeat with the second tag in the opposite direction, and pass both tags through the gap in the center in opposite directions. Lubricate and pull the standing lines to draw the wraps together into a neat barrel.
Tip Take the same number of wraps on each side, and lubricate well before you seat it. An even, well-lubricated blood knot cinches into a tidy, symmetrical barrel; uneven wraps or a dry knot seat poorly and cost you strength.