Since crutches are designed to support the weight of an adult human (~180 lbs, give or take), it goes to reason that their weight-bearing structure could be re-arranged into a chair. The crutches don't have enough wood in them to make the entire chair, so I dug some white oak left over from another project. I had three pieces,1"x3", about 48" long. I tried to waste as little material as possible, and re-used as many fasteners from the crutches as I could.
The design was derived by just cutting and hold pieces up,evaluating, then pre-drilling and screwing them together. Since I designed it as I made it, there are some things I would change if I were doing it over. For one, the dowels that run from the crutch armpit support back down to the feet like secondary legs could probably be eliminated if the front legs were pushed forward to sit at the very front of the horizontal feet. Then the feet would be long enough to resist the force of a reclining person from tipping the chair over backwards.
The angle of recline is rather severe; the back is also pretty short, providing only lumbar support. However, the chair is really very comfortable. Getting in and out is facilitated by the crutch armpit supports, which form handles to push up and off or a handhold for lowering yourself onto the seat. Overall, the front edge of the seat is about 14" off the ground, 26" to the top of the back, 16" wide, and weighs about twenty pounds. Both the seat an the back are 14" x 16".
All dimensions are approximate; since designed it on the fly, nothing was really measured. Everything can be roughly judged from the photos. Every pair of crutches is different, so you will have to experiment with your own materials. I used the oak because I had it; a number of materials could easily be substituted. After I built it, I took it apart, taped off the joints, stained and sealed it, then re-assembled, gluing the joints on the bare wood left by the tape as well as screwing them. Finally, I buffed in a coat of furniture wax.