What are the parts of a spring-joint caliper?
The legs of the caliper are joined together with a circular spring at the top of the tool.
Adjusting nut and threaded rod
When the adjusting nut is twisted, the nut and threading rod work together to compress or expand the spring which opens or closes the legs of the caliper.
The legs are the measuring faces of the caliper. The distance between them is adjusted when the nut is twisted.