This is an American Pheasant Tail nymph. Perhaps it's greatest strength is that it reasonably imitates a wide variety of mayfly species in their nymphal stage. It's a pattern no fly fisherman anywhere should be without.
If I were allowed only one hook size for a pheasant tail, it would be a 16 and a Dai-Riki #730 nymph hook makes for a well proportioned fly, which is very important for this pattern.
For thread, it's hard to go wrong with Flymaster 6/0 in brown. Start your thread on the hook shank leaving a hook-eye length space behind the eye and take wraps rearward before breaking or snipping off the tag.
For the rib, copper Ultra wire in the small size is a good choice. You can usually get 2 or 3 flies out of a 6-inch length. Secure the wire to the near side of the hook shank and take wraps rearward to the bend. You'll see why I like to keep it on the near side in just a minute.
Although natural colored pheasant tail works fine, feathers that have been dyed dark brown look really nice. Strip 8 or so fibers from the stem doing your best to keep the tips aligned. Measure them to form a tail about 3/4 of a hook shank in length. Secure them with 2 or 3 wraps of tying thread and then take a single wrap over just the hook shank and leave the thread right where it is. Get hold of the pheasant tail fibers and begin making adjacent wraps up the hook shank. Try to get them to start right at the base of the tail so you don't see exposed thread wraps. The hanging tying thread should help to hold the pheasant tail fibers together as you wrap them forward.
Once you've created an abdomen about 2/3 of a hook shank in length, tie off the pheasant tail fibers with a few firm thread wraps. With that secure, you can snip the butt ends off close, being careful not to cut your tying thread in the process.
Get hold of the copper wire and bring it toward you. By tying it in on the near side of the hook and making counter wraps with the wire, it means the first wrap contacts the bottom of the hook shank and doesn't jostle the tail around. With that complete, begin making open spiral counter-wraps up the hook shank to the front end of the abdomen. Firmly securing counter-wrapped tying materials can be difficult but if you use the material to temporarily change the direction of your thread wraps, like I've done here, you can make everything secure. When you're done, again use the material to change the direction of your thread wrap back to normal. This sounds more complicated than it really is.
Once this is done, you can cut or helicopter the wire off close. At this point it's critical to get your thread located 1/3 of the way down the hook shank from the eye. This will not only establish the length of the thorax and the wing case but will also aid in getting the legs to be the correct length. Be sure to take your time to get this location correct, it will be worth it in the end.
Fibers from the same pheasant tail are used to form both the wing case and the legs. This time strip 10 or so fibers free from the stem while keeping their tips aligned. Measure them so they extend from the back edge of the hook eye to the back edge of the hook bend. Getting this measurement right is critical to having the legs come out to be the correct length. This may seem a little odd, but you'll see what I mean in a minute. Transfer that measurement to the tie-in point, and with firm thread wraps, anchor the fibers to the top of the hook shank. Take wraps forward to build a ramp down to the hook eye. Lift the butt ends of the pheasant tail up and, using the hook eye as a guide, snip them off close and take a few wraps back down the hook shank.
Peacock herl is used to build the thorax of the fly. Two herls will build a good sized thorax on a size 16. Before tying them in, snip the brittle tip ends off square. Tie the herl in and take thread wraps all the way back to exactly that 1/3 thorax-wing case point. Again, leave the thread hanging right there, so it holds back the peacock as you make adjacent wraps toward the eye. Leaving a small space behind the eye, secure the herl with a few tight wraps of tying thread and then snip the butt ends off close.
Now comes the fun part. Pull the pheasant tail fibers over top of the herl to form the wing case. The tips should extend out beyond the hook eye. While holding onto the tips, take 2 or 3 wraps behind the hook eye to anchor the front of the wing case. You should be left with something that looks like this. Jam your thumb nail between the hook eye and the pheasant tail to push them up and back. Wiggle your thumb to get roughly an equal number of fibers on each side of the fly. When you're satisfied, pull the fibers down and back with your left hand and squeeze them tightly. You can then take thread wraps, starting just behind the eye and working rearward, to lock them in place.
You can see now why getting that tie-in length right is so critical to leg length. If everything looks good, do a 5 or 6 turn whip finish to secure the thread and build a small head on the fly. You can then snip or cut your tying thread free.
Head cement, run from the back of the wing case down over the thread wraps, not only looks good but really helps out with durability.
Although everyone seems to have their own preferences, these are the proportions I like on a pheasant tail. Tying a dozen or more at a time is a great way to achieve tying consistency and besides, in my estimation, you can never have too many pheasant tail nymphs.