I have made 5 corner chairs and this is what I did. The lower part of the top rail/crest (the part that connect the 3 leg posts) is jointed over the rear/center leg with a large dovetail joint. A large half lap joint will also work. The top crest is then glued and screwed to the lower piece. I make the tenon on the rear/center leg long enough that it goes completely thru the lower piece and into the upper piece.
I start by making the lower piece 1" large on all sides. Place it on top of the 3 leg tenons. Mark the center/rear leg tenon and drill this hole in the lower piece. Place this lower piece onto the rear leg tenon and mark the location of the other two leg tenons and drill these holes. Now place your template on the lower piece and shape it. Drill a hole in the upper piece and place the lower and upper pieces onto the 3 tenons. Mark/trace the upper piece from the lower piece and shape it.
Thanks for the detailed answer; a lot of wisdom therein.
I must say , I was surprised that you felt the need to leave a 1" trim margin on the lower piece. I would have imagined the three tenon locations were better determined than that, so I am sure there is something I am missing.
Thanks for the clarification. Since I was planning to make more than one chair, I will not make the lower piece template until after I have assembled the lower portion and see if I can save some of the highly figured tiger maple I plan to use.
My pattern making instructor said similarly; "measure twice, cut once."
Moving to another detail, most instructions I have found for offset turning of a cabriolet leg call for straight taper, but some older pieces seem to have something slightly more elegant in the manner of tapering of Greek columns.
After studying some photographs of early pieces very closely, it would appear that what was done was for the coincident point of the centered turning and the offset turning to be made to occur at some distance below the square portion. By eye, it looks to be about 1/3 of the way from the top, and it gives the legs a more substantial appearance. Anyone have any references about this. Most of the current information posted calls for the two to intersect at the bottom of the square pommel.