The Nail Knot is the best knot for joining the end of a fly line with the puff end of a fly leader. The knot is smooth, streamlined, and will run freely through the guides of the fly rod. Caution: This knot is designed for use with the modern synthetic fly lines; do not use it with an old silk fly line, because the knot will cut the line.
To Tie a Nail Knot:
Place the end of the fly line and the butt end of the leader - pointing in opposite directions - along the length of a tapered nail. Allow sufficient overlap.
Wrap the leader five or six times around itself, the nail, and the fly line, keeping windings up against one another. Run the butt end of the leader back along the nail, inside the wraps.
Pull both ends of the leader tight, and then remove the nail and tighten again by pulling on both ends of the leader.
Pull on both the line and the leader to test the knot, and clip off the ends, completing the knot.