The three replaceable carbide cutters can be rotated to a fresh edge and replaced when all edges are dull. The width of the scribe is set by turning the threaded shaft and raising or lowering the cutters in relation to the foot--shown here with a non-marring cover attached. A lock nut retains the setting.
The Thingamejig works best on uniform surfaces. Here's it's being used to scribe a laminate countertop to a wall. The cutters produce a deeply scored line that it easy to see and prevents delicate materials from chipping or splintering.