Choosing the correct fishing line is of fundamental importance in the sport of fly-fishing. By “reading” a river fly-fishermen are able to determine if the fish are feeding at the water’s surface—where floating lines are utilized—or deeper inside the river current and even on the river bed—when a sinking line is required.

Artificial, nearly weightless flies are attached at the end of a fishing line. This may be a dry fly, wet fly, or “streamer” fly and their size is what determines the number classification of fishing line required. A fishing line’s number, in turn, then determines the fly rod’s number.
When all of these parts are matched correctly they create a well balanced fly-fishing outfit.

If a fisherman intends to fish with a dry fly then a floating line is necessary. On the other hand, when using a wet fly, nymph, or streamer then a sinking line is required to help the fly imitate the sinking action of its real-life example.

Sinking lines sink at a predetermined rate of meters-per-second. A fast sinking line is used when a heavy river current requires that a line be sunk quickly. If a river is slow moving then a slow or medium speed sinking line is sufficient.

Because the fishing line is the fundamental actor, indeed the protagonist in the sport of fly-fishing, choosing the correct line of good quality is vitally important.