Thin, strong line (usually Dacron or gel-spun) that fills the fly reel under the fly line and gives you extra yardage when a strong fish runs past your fly line.
When to use it
Under fly line on the reel
Extra line for long-running saltwater fish
Strengths
Adds capacity for big runs
Builds up arbor for faster line pickup
Weaknesses
Only relevant to fly setups
Too little backing loses trophy fish
Key facts
Best species: Saltwater fly targets — bonefish, tarpon, redfish
Knot notes: Arbor knot to spool, Albright/nail knot to fly line
Common mistakes
Skimping on backing for bonefish/tarpon (200+ yds recommended)
