The Pheasant Tail (or "PT") is the nymph equivalent of the Parachute Adams — generic enough to imitate the nymphal stage of almost any mayfly. Sizes 14–20 cover most water. A bead-head version sinks faster and adds an attractor flash.
It's the workhorse of indicator and Euro nymph rigs alike. When you don't know what's hatching but fish are eating sub-surface, tie on a Pheasant Tail.