The Nail knot is used to tie fly-fishing line to a leader. It's been used for a long time and is a proven way to attach the two types of line. It takes some practice to learn and a hollow tube like a small straw is easier to use than a nail when tying this knot. Although it has been replaced by metal eyelets in many cases, this knot is still the best way to attach the line to the leader.
When to Use the Nail Knot
Any time you need to tie a leader to a fly-fishing line use the Nail knot. It makes a smooth connection that will not bind on the guides as it goes through them. This knot will also hold under pressure when fighting a fish.
How to Tie the Nail Knot
To tie a Nail knot:
Use a nail (that's how the knot gets its name) or a small straw used to stir coffee.
Lay the nail (or straw) beside the end of the fly line and put the end of the leader beside both of them. Leave at least twelve inches of leader line above the nail to use it to tie the knot.
Hold the nail, leader, and fly line together with one hand and wrap the end of the leader around them six to eight times.
Make the wraps close together and keep them even, working toward the end of the fly line and toward the leader.
Push the end of the line you're wrapping back through the straw or beside the nail and then pull on it to tighten up the loops of line.
Slide the nail out of the loops and then pull both ends of the leader line at the same time to snug it up.
Pull hard on both ends of the line to tighten it up on the fly line; then trim off the end of the tag so it doesn't stick out.
Pull on the leader while holding the fly line to make sure the line is strong and doesn't slip.
Tying a Nail knot.
A finished Nail knot.
This knot will not hold for any purpose other than attaching leader to fly line. The fly line is soft enough to hold it but others will not. Use a Blood knot or two Uni knots when tying other types of line together.