Backing
Thin braided line wound onto a fly reel underneath the fly line, to fill the spool and give you reserve line on a long run.
Fly line is only about ninety feet long and quite thick. Wound straight onto a bare arbor it would coil into tight loops that never straighten out, and it would leave the spool half empty so every turn of the handle picked up almost nothing.
Backing fixes both. Twenty to thirty pound Dacron fills the spool so the fly line sits at a larger diameter, which means larger, lazier coils and a faster retrieve. It also means that when a fish runs past the end of your fly line you still have a hundred yards of something attached.
For trout on a 5-weight, running out of fly line is rare and the backing is mostly there to fill the spool. On bigger water it is the part that saves the fish.
Most fly shops will spool backing and line for you when you buy a reel, and it is worth letting them. Getting the fill level right by hand is fiddly.
Related terms
Where this comes up
The guides that use backing and show it on real gear.
Backing is one of 206 terms in the glossary.
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