Glossary
Backing
Thin braided line spooled onto the reel under the fly line to add capacity for a long, fast run.
Backing is thin, strong line, usually braided Dacron or gel-spun material, that gets wound onto the reel spool first, with the fly line tied to the end of it. Fly line itself is only around 90 to 100 feet long, which is not enough to handle a fish that runs hard, so backing fills out the reel and gives extra length to fight with.
How much backing a reel needs depends on the fish being targeted. Small stream trout rarely take out much line, so 50 to 100 yards is often plenty, while bonefish, tarpon or big steelhead can strip off far more, which is why saltwater and big-fish reels are built with larger arbors specifically to hold more backing and pick up line faster.
Backing is usually connected to the fly line with an Albright knot or a loop-to-loop connection, and it is worth checking that connection periodically, since a knot that slips is one of the few ways to lose a big fish outright. Buying a reel with too little backing capacity for the intended species is a common and avoidable mistake.