While there are many ways to attach backing to the back end of a fly line and the butt section of a leader to the front end of the fly line, one of the more popular ways for both is to create loops at both ends of the fly line. Most fly line manufacturers today incorporate factory-welded loops at both ends, which is no doubt convenient. However, if for some reason one of these loops breaks or you prefer to make your own loops, knowing how to tie a nail knot will come in handy. Several companies specializing in fly-fishing accessories produce nail-knot tying tools that many find easy to use, but nail knots can be tied without these tools with some practice. When creating loops in fly line, use at least two nail knots and don't use monofilament that is too heavy. I prefer to use 12- to 16-pound test — thinner- diameter monofilament has the ability to slightly dig into the coating of the fly line when cinched, which is a good thing.