These micrometers come with a calibration certificate traceable to NIST that states they've passed a test for accuracy. Change out the steel extension rods supplied in 1" incremental lengths to measure the depth of holes, slots, shoulders, and projections. The thimble has a constant-force mechanism, so that once the spindle contacts the surface of the measured object, it will stop tightening. This ensures repeatable measurements and prevents over-tightening.