Pierre Vernier

In Brussels, in the year 1631, Vernier published his treatise La construction, l'usage, et les propriétés du quadrant nouveau de mathématique, and dedicated it to the Infanta.

[1][2] To a quadrant with a primary scale in half degrees Vernier proposed to attach a movable sector, thirty-one half degrees in length but divided into thirty equal parts (each part consisting then of a half-degree plus one minute).

Christopher Clavius had earlier mentioned this idea but had not proposed to attach the scale permanently to the instrument.

[3] The name vernier is now applied to the small movable scale attached to a caliper, sextant, barometer, or other graduated instrument and was given by Jérôme Lalande.

Lalande showed that the previous name, nonius after Pedro Nunes, belonged more properly to a different contrivance.

A Vernier caliper using the scale invented by Pierre Vernier
Vernier caliper scales; main at top, vernier at bottom. It reads 3.58 mm ± 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 the rightmost graduation that is 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 accuracy and is the "Vernier constant" for this scale.