Many years ago, I saw Tage Frid demonstrate how to use a jig like this for machining curves - and I had to wrack my brain to remember how he put the jig together.
There wasn't anything available on the web to jog my memory, so I pulled out a few of his old books, hoping I would find something. No luck.
So I sat down with my sketchbook and tried to come up with something that would work, based on what I'd seen him do 25 years ago... and I actually was able to remember most of it.
That's pretty good, coming from someone who can barely remember what she wore yesterday, or had for dinner two nights ago.
Here's the jig with the seat back held firmly in the center.
It's sort of a rite of passage to build the stool that Tage Frid discusses in his Furniture making books, so I built this set a long time ago.