This fly is a "standard" downstream winged wet fly with the wings set down.
These sort of "Standards" have mainly been propagated by professional fly-dressers and many books, over the years, for various reasons. Some apparent and sensible, and some less so, this does not necessarily mean they are the best, or even useful at all in some cases!
Set Down.

Some commercial flies I have seen , especially of this pattern, are absolute abominations, the bodies formed of olive floss and not waxed at all, the tying thread is often black, the thing is then about as translucent as an elephant painted black, and about as delicate.
Set down means that the natural curve of the quill feather wing slips is towards the hook shank, and the wing slips are placed front to front with the shiny side of the feather outside, partially cancelling out the natural curve of the feather, and holding the wing slips together.The wings slant almost parallel to the hook shank, aided by a bed of silk at least the same height as the body windings. The slant of the wings may be controlled by position and height of the wing bed when tying in, and may be varied somewhat if desired. Wing slant or “set”, also varies considerably. Many Scottish dressers use a much steeper wing angle on their flies, usually about 45°.
This winging style has become a sort of standard for many winged wet flies, and these flies are also often hopelessly overdressed, meaning far too much material is used. The imitation value is nowhere near as good as many other styles, although better when kept sparse and neat. The whole fly has a streamlined appearance, the hackle is also tied in so that it slants back at about forty five degrees to the hook shank and usually long enough to reach the hook point and even cover it slightly, although this is again purely a matter of taste, and dependent of course on the size of hook and length of shank.
This fly when dressed correctly will have a good "entry" when cast, meaning it will not cause too much disturbance when it lands on the water, and will sink fairly well, and will cause little disturbance when held in the current, or indeed when "worked," and will maintain its shape fairly well even in turbulent water. The hackle is hen. The wings are tied in last and then the head formed over the roots. The fly really depends on being moved by the current or worked to resemble a living creature, this is unnatural for a dead winged fly imitation, and is basically not particularly well suited to upstream or dead drift fishing, although it may occasionally work for this purpose. In larger sizes it is a good fry imitation. The shape of the wings is in my opinion largely due to a series of mistakes made by various authors, but that requires a separate article and explanation of it's own.
Set Up.

The wings on this fly are set up, and the hackle is not swept back to the same degree. Set up means that the wing slips are tied in with the natural curve of the feather curving away from the hook shank. The slip wings are set at an angle of about 45 degrees, and stand well proud of the body, this angle may be varied. This fly may also be used for downstream fishing, but heavy currents or "working" the fly will cause the wings and hackle to be compressed towards the body of the fly and force it out of shape. The imitation value is then usually not particularly high especially when overdressed, and many people think these flies are taken as small fish or active nymph imitations rather than as imitations of drowned winged insects. They sometimes work well though, and many such patterns have long traditions.
This is a fairly typical upstream wet fly.
The wings are tied in last, and then the head formed over the cut roots. This fly also mostly depends on being worked to resemble something alive, and is also not very suitable for upstream or dead drift fishing although it is somewhat better than the first example. The hackle is hen fairly short and sparse, with the majority of the hackle beneath the hook. A beard hackle (bunch of hackle fibres) may also be used.
An “Advanced wing” style was also popular on a number of dry flies. The most well known of these are probably the Mole fly and the John Storey, but there were quite a few others.
Set Up back to back.

This fly has slender wings, and the slips are not only “set up”, but tied in to curve up and out away from the body of the fly, “back to back”, with the shiny side of the feather on the inside. The hackle is only slightly brushed back when tying in, and the body is kept short and thin. The wings are tied in before the hackle! and the hackle covers the wing roots. The whole appearance is delicate and the fly is a fairly accurate imitation of a living creature. As such it may be used as an upstream or dead drift fly with some success. The Hen hackle is fairly short, sparse, and mobile.
Upright slip, split or bunch.

The fibres are tied in either as slips or in a bunch, and then split.The body is then formed, and the hackle applied last. The appearance is neat and delicate, with a high imitation value. The hackle is hen. This fly is excellent for upstream or dead drift work. It will also work as a downstream fly. Soft translucent feathers are best for these bunch wings. Jay primary or secondary, was at one time very popular for this, as were various duck body feathers. For the greenwell hen blackbird was used, but one may also use starling and other feathers. For small imitative flies fine fibred translucent feathers work best.