Rolland,
I'm asuming you are referring to housings that start flush to the face of the morticed timber at the top of the mortice. I usually refer to these as diminished housings. Diminished housings are most often indicative of scribe rule framing. They are found occassionally in square rule frames, but they typically exhibit square housings. Interestly in German origin framing a diminshed (haunched) housing is often used on only one shoulder of a joint while the other shoulder is left square.
Square housings pretty much gaurantee you are looking at 19th century framing, while diminshed can be as old as the colonies.
Hope this helps.
Rudy
