Besides dividers and straightedges and squares they also used sectors. I think what you show at the beginning of the video as a square is actually a sector.
Thank you for this very insightful illustration on the original process. I tried to follow your approach but I’m not really clear on the placement of the corner circles. Can you elaborate where and how the centers of the corner circles are placed?
First, let me say I'm not a luthier, although I work in wood. I'm wondering if we modern people are over-thinking the processes of the old masters? Given the slight differences in shape between makers, is it possible that they arrived at the pleasing shape by freehand, and we are extrapolating geometry from that to give more kudos (and repeatability) to their art? I obviously haven't done the background research that you evidently have, but I'd be interested to know your thoughts, David.
Of course you can make any shape out of circles, if you are willing to work hard enough at it. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-qS4H6PEcCCA.html
There are some deep questions hiding in your question. Many curve families can trace the historical instruments. What distinguishes a circle geometry approach is that a simple a logical choice of parameters will generate the full range of Cremona instrument shapes seen. And, more and more accurate shape matching leads to simpler governing parameters. This is the opposite of what happens with a forced fit. No. It is not reasonable to believe the actual instruments match the predictive geometry so well by accident. They match well because however they actually proceeded, it was basically homomorphic to my description.