The Robert Sorby Vessel Caliper was specifically designed for internal and thin wall measuring for bowls, urns, goblets, etc.
The straight arm of the caliper makes it especially useful for checking narrow vessels and sizes can be read at either end.
Calipers have plated steel legs for corrosion resistance and have a friction fit adjustment point.
The size refers to the approximate length from the tip of the straight leg to the center of the set screw. Available in 2-1/2-in, 5-in and 10-in