The leader is the nearly invisible section of nylon material that connects the bulky fly line to the delicate fly. The leader is fairly heavy where it connects to the line (this is called the butt section), and then tapers down in weight and thickness until it reaches its end where you tie on the fly. The main idea here is stealth — you want the typically 9-foot-long leader to gently present the fly without spooking the fish. You want to make the fly look like it landed on the water all on its own. The leader also completes the transfer of energy from the line to the fly during your cast — it helps your line roll out and then straighten for an accurate fly presentation to the fish. You're going to clip sections of the leader as you adjust flies so, unlike your fly line, you'll be changing out your leader pretty regularly.
When buying a leader, keep it simple: Buy a knotless tapered nylon leader that matches your line weight. "The most critical piece and also the least expensive is the leader," Rosenbauer says. "It's the link between the line and your fly. I would rather have the right leader than the right fly. If you give me the right fly with a leader that's too stiff or clunky, I'm lost." Leaders are designated in size by 'X' ratings: ranging from heavy 03X leaders for saltwater game fish to 8X leaders for panfish. For trout, buy a 9-foot, 5X knotless tapered leader like the Mirage from Orvis. [$28; amazon.com]