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.

One of the more confusing aspects for newcomers to fly-fishing is learning the different fly lines and their purposes. A cautionary note to beginners: Don’t let this intimidate you and keep you from ...