This photo is a section of white oak, riven and planed into apron stock for a joined stool I am making. The front, true face is to our right, and the tenon is laid out from this face. The other day I was writing some text for the never-ending joined stool book Alexander & I are doing. The subject was cutting tenon shoulders in this sort of stock . In some cases, this stock is planed flat on its face, with two edges (mostly) square to this face, but with the back face left irregular and even unplaned. Working with this tapered cross-section material can get confusing. To saw the front shoulder, the stock is sitting on its irregular rear face. This cants the stock a bit. It’s important to remember that the saw’s teeth should be parallel to the tenon line struck with the mortise gauge.
In this photo I enhanced the gauge lines with a pencil (some will know how it pains me to write that) so we can see what’s what. The stock is canted a bit, so the saw’s teeth should be as well. You have to think it through when you pick up the saw; the tendency is to saw with the teeth parallel to the workbench, which with squared-up stock is essentially the same as the teeth being parallel to the rail’s face.
It becomes simpler to see and feel when cutting the rear shoulder of this rail. Now the stock is laid on its front true face, thus it does sit flat on the wooden bench hook.
But the rear face, where the saw enters the work, is quite canted. It’s easy to make a mistake and run the saw parallel to this irregular face…which would ruin the workpiece, by cutting into, or even through, the tenon. See how the teeth are parallel to the scribed lines, not to the surface of the stock.
There’s lots more to the configuration of this tenon, but all I was after today was this point about the effects of the cross-section on layout and cutting. A small, but important point when working with riven stock.