An in-plane loop keeps the fly leg and the rod leg in the same plane. An out-of-plane loop does not: the fly leg travels in one plane and the rod leg in another.

That is a different question from width, and a different question from fly-leg shape. Narrow and wide describe how far apart the legs are. Concave and convex describe how the fly leg curves. In-plane and out-of-plane describe how the two legs sit relative to each other in space — and a loop can be perfectly narrow and still be out of plane.

Keep the categories apart when you answer. A tailing loop is a loop with a concave fly leg that dips toward and crosses the rod leg. A trailing loop crosses the rod leg without a concave fly leg. Neither definition says anything about plane, so “the legs crossed, therefore it was out of plane” is not an answer.

On the lawn, sight the cast from directly behind or above the rod. From the side, the two planes can overlap and the fault is easy to miss.

Source

From the FFI Fly Casting Definitions (August 2025); terms follow the FFI Fly Casting Definitions (In-Plane Loop, Out-of-Plane Loop).