The Yellow Sally is tied to imitate the adult stage of the stonefly insect known as the Isoperla, however I’ve found that it also works as an effective attractor pattern, even when the Isoperla is nowhere in sight.
This is the version I most commonly tie, though there are many variations that work just as well. One thing that I’ve found key is to make sure that the body is dubbed very sparsely so that the red thread comes through when the fly is wet.
Fulling Mill FM50 55
6/0 UNI-Thread Waxed — Red
Rooster hackle — Grizzly
Deer Hair — Bleached
Kapok Dry Fly Dubbing — Fluorescent Yellow