It's interesting to browse through Nicholson's book, which is available on Google Books here One of the surprising things about it is the extensive section on geometry. It is quite sophisticated and technical. Even if a joiner didn't have a formal understanding of geometry, having the ability to make use of geometrical concepts could be extremely useful. If you scan the last part of the geometry section, you'll see what I mean. I am wondering how common it was in the early nineteenth century for woodworkers to have this skill. Anybody know?