Straightness is only half of the 180-degree principle. The other half is alignment: at the start of each casting stroke, the intended rod tip path and the fly line must be closely aligned, laterally and vertically, with the target. A tip path can be perfectly straight and still be aimed along a different line from the one the fly line lies on.

When that happens, the straight path does the damage. The tip pulls the line out of its old direction and into the new one; energy that should have gone into unrolling the fly leg goes into changing its direction, and the fly leg curves instead of running straight. The tip path was straight. The cast was not.

So check the line before you change the stroke. Is the back cast lying in line with the forward path? Is the line off to one side as the stroke begins? A straight tip path is necessary, not sufficient: it also has to start on the line the fly line is already on. Fix the alignment and the same straight path starts producing a straight fly leg.

Source

From the FFI Fly Casting Definitions (August 2025) and the mechanics discussion in the MCI Study Guide (March 2023); terms follow the FFI Fly Casting Definitions; exam details are in the CI Study Guide (2025).