Have you come to a rive or stream and don't quite know what fly to put on?  You think you might use an attractor fly with maybe a bead head dropper nymph.  That may be the right way to go, but what kind of attractor fly should you use there are so many.  One way to help you choose is to ask yourself what kind of river is it.  Is it primarily a Mayfly river, Caddis river or both?  If it's primarily a Mayfly river then you should go with a attractor Mayfly pattern such as an Adams or Parachute Hare's Ear.  If it's Primarily a Caddis river then you should do the same with Caddis attractors such as an Annihilator.  If both Mayfly and Caddis fly are out and about then  you should trade off until on or the other becomes consistent.