The Rio Grande has been a standard dry fly pattern for a long, long time. I suspect the it was designed as a quill wing sometime around the Royal Coachman, and was modified into a hair wing about the same time that the Wulff patterns became popular. "Truce" simply implies that the wing style is back over the body instead of straight up.
This pattern has become popular, so I'm told, on B section of the Green River. I have fished it on occasion as an attractor and found it to be quite effective. I do like the way it looks tied on a straight-eyed hook, and prefer the hackle to be sized one size smaller than the hook.
Hook Dai-Ichi 4X Long Straight eye #1750
Size: 8 to 12
Thread: Brown 6/0 or 8/0
Tail: Golden pheasant tippet
Body: Small black chenille or black ultra chenille
Wing: White calf tail
Hackle: Coachman brown
Step 1: Tie in a small bunch of golden pheasant tippet for the tail equal to 1/2 the length of the hook. Tie in and wrap forward the small chenille for the body. The body should take up about 2/3 of the fly.
Step 2: Select and clean a bunch of calf tail. This should be stacked to even the tips and provide a more definite silhouette. Tie this on top of the hook taking care to keep it on top, or do not allow it to roll down the side of the hook.
Note: When tying in a truce wing, it is important that you trim the bunch of hair on an angle parallel that of the wing. This will allow the fibers on the bottom to be secured as well as the fibers at the top of the bunch. In other words, each hair fiber has some contact with the tying thread. This will also allow for a more gradual elope on which to tie your hackle.
Step 3: Once the wing is secure, cover the ends with the thread. This will make a suitable surface to wrap the hackle, and make the hackle less likely to slip when you wrap it. Next, tie in several good hackles to fill in the remaining 1/3 of the body.
Step 4: Wrap the hackles forward one at a time, leaving a small space in between the wraps. This will allow you to bring the second hackle forward and not tie down as many of the hackle barbs. Make a full thick hackle collar, similar to a Sofa Pillow pattern. The head should not be overly large or even noticeable. This is where using an 8/0 thread will help you tying.
Whip finish and cement.