A narrow loop comes from a relatively straight rod tip path, a wide loop from a convex (domed) path, and a tailing loop from a concave path that dips downward during the casting stroke.

The rod tip path during the casting stroke determines what the fly leg of the loop will look like. Match the casting arc to how much the rod bends, and the tip travels straight — that is the narrow loop you want most of the time.

• Narrow loop — straight. The casting arc matches the rod bend, so the tip path is a straight line and the fly leg is straight.
• Wide loop — convex. The casting arc is too wide for the bend, so the tip wanders up and over in a dome shape and the fly leg curves upward.
• Tailing loop — concave. The casting arc is too narrow for the bend, or force was applied unevenly, so the tip path rises during the stop, dips, and rises again. The fly leg dips and crosses the rod leg.

Read the fly leg and you know which path you made: straight, curve up, curve down.

Source

From the CI Study Guide (2025) (PDF), Bruce Richards’ Five Essentials and Task 16 loop shapes; terms from the FFI Fly Casting Definitions.