Trammels can be used lots of ways , like inside and outside calipers , hermaphrodite calipers also. With combinations of ball on one and scriber point or in or out caliper leg. If you need to layout bolt hole circles and have a start hole the ball end would be centered using the ball the other can be set to scribe or to measure between holes . They use to sell a vernier style trammels but with a good scale you can locate within .005 really easy. Extremely versatile tool ,but with dros there almost of no use. But for me I can say ill keep my set ill know when I do a layout where everything goes. I used mine on many 12foot dia. Tanks and lids , even on missiles for the government , u2 style rockets were fun to build . There lugs were titanium with specified finishes. I think about two thousand at a time . Even the fins had tight specs. YUPP I learned well to use trammels l sure wish I could go back in time just to thank the old guys who taught me. Don't think everything new is always best , old may still be better. Of course you have to use the brain to work them not push a button.