The backing is an added braided line that connects the reel’s spool to the ordinary fly line. Since fly lines average 75-115 feet in length, the lines require backing for those times when a large fish runs out all of the regular fly line. Without backing, you would quickly part ways.
Small waters rarely require much backing; usually fifty yards is adequate. In larger waters, at least 100 yards of backing is needed. If a fish runs out more than 100 yards, your chances of landing it are slim. Saltwater species require 200 to 600 yards of backing. In the open ocean, however, there is less chance that the fish may wrap up line around a rock or a log.
I prefer twenty pound braided Dacron for fresh water conditions and thirty pound braided Dacron for saltwater conditions. I prefer the braided Dacron because its diameter is larger than the new Spectra lines. Thin Spectra lines can cut into the spool of backing and jam; this usually causes a big fish to be lost. Flyfishers use an arbor knot to connect the backing to the spool. The fly line/backing connection requires an Albright knot.
back·ing / ˈbaking/ • n. support or help: he accepted the backing of the police group. ∎ a layer of material that forms, protects, or strengthens the back of something: the fabric has a special backing for durability. ∎ (esp. in popular music) the music or singing that accompanies the main singer or soloist: the trio provided backing to some of the most popular vocalists of the day.