In theory, fly-fishing is a simple sport: Pick a body of water, choose a fly-fishing rod, select your “fly” (or bait), tie a secure knot, cast your line and, hopefully, land a fish on the other end.

There are a lot of numbers in fly fishing, and some of the numbering may not make sense to the uninitiated—or to the initiated, for that matter. A higher number means a smaller hook, yet the opposite ...