What it does: Used to attach leader butt to fly line or to attach backing to fly line
Why it's useful: The most popular knot for attaching leaders and leader but to fly line, the nail knot has a very smooth, low profile that allows it to slide easily through guides. Though it doesn't necessarily require a nail, you will need a tube of comparably narrow diameter to tie it (e.g., a particularly thin ballpoint pen). The nail knot is moderately complex, and not recommended for big fish as it relies on the fly line's coating for strength, but few other knots give you this smooth a connection.
How to Tie It:
1.Hold the nail/tube and the end of the fly line between the left thumb and forefinger ofyour lefthand (with roughly 2"of the fly line and the tube sticking out). Take the heavier end of your leader in yourright hand and form a 2" loop around the nail and fly line. Pinch the loop down with your left hand, then wrap the leader 5 more times around the line, leader, and nail. Do it tightly, so that the loops butt up against each other, and be sure to leave 1/2 to 3/4" of the end sticking out.
2.Grab all the loops with your left hand, then carefully slide the end of the leader back into the opening, running alongside the nail. Then carefully remove the nail.
3. Pull gently on both ends of the leader (don't pull the fly line!) to partially tighten the knot. Moisten, check the wraps to be sure they're smooth, then fully tighten the knot by pulling on the leader until it "bites" into the fly line. Finally, "seat" the knot with one hard pull on the fly line. Trim the ends, and you're done.