John Aniano in central NJ
Wiley, et. al.,

Something I've been thinking about, what with all the mortise chisel discussions now. I can't quote any historical source, this is just an observation from cutting 16 mortises when making a joint stool...

If one assumes that the bevel is ridden downward into the mortise, if the edges of the chisel face are sharp, they would diagonally scrape/pare the sides of the resulting mortise as it cuts. If the chisel cross section is slightly trapezoidal, i.e., wider at the face than at the rear, this would facilitate this diagonal paring/scraping action.

Conversely, if the chisel is driven directly downward, there would be less or no paring as described above that would be possible because of the geometry. Again, if the cross section is trapezoidal, this side taper would preclude the paring/scraping action.

Just my two cents...

John