VERNIER

noun a short scale made to slide along the divisions of a graduated instrument, as the limb of a sextant, or the scale of a barometer, for indicating parts of divisions. it is so graduated that a certain convenient number of its divisions are just equal to a certain number, either one less or one more, of the divisions of the instrument, so that parts of a division are determined by observing what line on the vernier coincides with a line on the instrument.