The Prince Nymph is a contradiction: it doesn't really look like any insect that exists, but trout eat it year-round on almost every river. The bright white biot "wings" on top of the dark peacock body give the fly its signature contrast — invisible from above (riverbed colors), bright from below (where a feeding trout looks).
Sizes 12-18 cover most water. Bead-head versions sink faster and add a second attractor element; the standard "searching" prince is the bead-head in tan or gold.
Tie it as the lead fly when you don't know what's hatching, with a more specific imitation as the dropper. The pattern's job is to find the player; the dropper closes the deal.