Many schools of thought and much of it differs by area. Alot of guys use three nymphs, some heaviest at the tail (point fly), others heaviest at the front to drag the others through the water column (difficult to cast though). Some use a dry, then a nymph on a dropper, some use a nymph then a buzzer. Some tie the dropper from the bend of the hook shank, others through the eye. As I said, there are many options. I personally like to fish a team consisting of a large dry fly on top (size 10 moth), a very small buzzer in the middle (size 18 horsehair) and a PTN on point (size14). All are tied through the eyes and the PTN is heavy whilst the dry is treated to float.