The side seat rails are angled up one inch and out two inches, the reason I am saying they have "dangled" tenons (double-angled). A little trigonometry told me that the rails have to be angled up 3.25 degrees and out 6.5 degrees, interesting but not all that practical. The right way to go is to set two sliding bevels to the correct angles by using a stick cut to the length of the rail placed on a surface with an accurate ninety degree corner. You measure up the correct amount from the corner to form a right triangle and then use it to set the bevel. I did that and could mark out the shoulders and tenons accurately, after a silly mistake I will tell you about below.
By the time the FDR chair was being constructed, I am sure there were workmen in the Portland WPA woodworking shop who could easily saw these out accurately and have them fit off the saw, but I'm not there yet, so I needed a different approach. In a way, cutting the shoulder of a dangled tenon is like cutting a pin and a tail simultaneously on a very thick board, because the cut angles both horizontally and vertically. I could see two approaches. The first way would be to saw them out roughly and refine them with a chisel. On a whim, I tried the only Japanese pull saw that I own to see if I could saw more accurately to the line. Somehow, pulling the saw along the knife line was easier for me, though perhaps it was because of the very fine teeth. To do a fair comparison between western and eastern saws, you would need to use a western saw with the same ppi, which I don't own. Dunno, but I used the Japanese saw from then on, with reasonably positive results. I sawed out the cheeks and fine tuned them with a chisel on a practice piece.
While I was congratulating myself on a good test piece, I decided to check to make sure that I had gotten the angles right and, to my intense annoyance, I found that the rail angled out correctly but it angled up far more than one inch! After spending time checking my sliding bevels and scratching my head, I realized what had happened. 3.25 degrees and 6.5 degrees (actually 93.25 and 96.5 degrees) are hard to tell apart by eye and one rosewood bevel looks pretty much like another rosewood bevel!
As I said, the results were pretty good and, with patience, I could refine them to get a pretty good result I thought. Then it occurred to me that I could make a guide to cut the shoulders very accurately and cleanly. Worth a try I thought. It worked well and I ended up with shoulders off the saw.
As you can see, my dog is not impressed.