Vernier scale

Vernier caliper scales; main at top, vernier at bottom. It reads 3.58 ± 0.02 mm by adding 3.00 mm (left red mark) on the fixed main scale to vernier 0.58 mm (right red mark). The main scale reading is that to the left of the zero on the vernier scale. The vernier reading is found by locating the best aligned lines between the two scales. The 0.02 mm engraving indicates the caliper's readability and is the "vernier constant" for this scale.

A vernier scale (/ˈvɜːrniːər/ VUR-nee-ər), named after Pierre Vernier, is a visual aid to take an accurate measurement reading between two graduation markings on a linear scale by using mechanical interpolation, which increases resolution and reduces measurement uncertainty by using vernier acuity. It may be found on many types of instrument measuring length or measuring angles, but in particular on a vernier caliper, which measures lengths of human-scale objects (including internal and external diameters).