To use, open the caliper by just pulling the legs apart (having the spacing nut between the legs rather than outside them allows this), put in position and then set the spacing nut to the wall thickness. Open calipers, remove and measure tip gap.
The spacing nut can be adjusted for any thickness from 0" to 13-1/2" on the 16-1/2" model and 0" to 5-1/2" on the 9" model. The respective throat depths (from nut to tip) are 11-1/2" and 5-1/2". These are equally good for dimensional take-offs of almost any oddly shaped object. Steel with brass fittings.
Made in France for Lee Valley.