One of the classic shapes for the seats of chairs or stools is the D shape. If you make or appreciate Welsh chairs (like I do), it’s a shape you see a lot. Yet many beginning chairmakers fret over making a D-shaped seat of their own dimensions.
I admit that when I started making chairs, I was similarly befuddled and preferred to trace the shapes of old seats or work from patterns drawn by other chairmakers.
That’s because the seat’s geometry eluded me. Was it a true ellipse? A false ellipse? Something else mathematical?
As with most things in woodworking, the answer was incredibly simple: It’s a half-circle married to a rectangle. Here’s how to lay it out.
Most modern chairs (for modern bottoms) are about 19” to 20” wide on average. The depth can be anywhere from 14” to 16” or even 17”.
So you start with the width of the seat. It’s 20” so the half-circle portion of the seat has a radius of 10”. Set your trammel points to 10”.
A simple half-circle seat isn’t deep enough – 10” in this case. Your butt would slip off it. So you need to make it deeper. Again, start with the width of the seat (20”) and that’s the width of your rectangle. The depth of the rectangle is some number that will get you someplace reasonable for the American posterior. In this case I decided the seat as a whole should be 15-1/2” deep so I made my rectangle 5-1/2” deep.
So I laid out a 5-1/2” x 20” rectangle on my seat stock. I found the center of one of the 20” segments. I put a trammel point on that centerpoint and drew the half-circle for the back of the D. Done.
— Christopher Schwarz