Backing does two jobs: it builds up the spool and it provides a reserve of line. It is the thin, strong line wound onto the reel under the fly line. Winding it on brings the spool up to a diameter that suits the fly line, and the same line gives a running fish something to take when the fly line is already off the reel.

• A. It replaces both leader and tippet
• B. It builds spool diameter and provides reserve line
• C. It makes every fly float
• D. It is the normal casting head

Answer: B. Neither job belongs to the leader or the tippet. Backing is not the connection to the fly, and it is not the line you cast — the fly line does the casting. Its place is under the fly line, on the reel.

That is why backing capacity belongs in the reel choice. A reel that holds plenty of backing is not just a bigger container; it lets a fish run without taking the fly line down to the knot. Fill the spool as the reel and line call for, and check the backing-to-fly-line connection before the season starts.

Source

From the FFI tackle glossary; terms follow the FFI Fly Casting Definitions.