Hi Northern Hewer,

I learned about the scribe rule compass settings from Richard Babcock and pictures in his books from very early structures (1600s, 1 possibly 1500s) he restored.

These really early buildings were laid out using scribe rule. Each beam (size), brace, mortise and tennon were laid out using chalklines (charcoal lines), plumbbob, and compass (divider). The compass would be set for different circles for different layouts. The master builder would scribe these circles as a reference so that when for instance the same size tennon needed to be laid out again there was a reference to set the compass (divider). Yes, you would still have to test fit joinery but it's really an amazingly accurate way to layout joinery.

Anyway these "master settings" were scribed onto one of the principal timbers as a reference, usually as a series of concentric circles. These were used not only in building the structure but also if repairs were needed and a timber needed to be replicated.

You can also layout all your angles using a compass, i.e. perfect 30, 45, 60,90 angles. To square up two beams to be joined at 90 a 3,4, 5 right triangle was used, the lenght of each side corresponding to 3,4, and 5 "turns" of a particular compass setting.