Vernier Caliper

A vernier caliper measures with two jaws -- one fixed, one sliding along a graduated main scale -- and a secondary vernier scale on the sliding jaw that lets the reading be interpolated far more precisely than the main scale's marks alone allow, traditionally down to 0.001". Digital calipers do the same mechanical job with an electronic readout instead of a vernier scale to read by eye.

It's more precision than most woodworking calls for -- a tape measure is plenty for cutting a shelf to length -- but for anywhere an exact diameter genuinely matters, like matching a dowel to a drilled hole, checking a router bit's actual diameter against its stated size, or verifying a tenon's thickness before a test fit, a caliper measures directly rather than relying on a rule's much coarser markings.