The Vernier caliper is an extremely precise measuring instrument; the reading error is 1/20 mm = 0.05 mm. Close the jaws lightly on the object to be measured. If you are measuring something with a round cross section, make sure that the axis of the object is perpendicular to the caliper. Measuring more precisely than could be done unaided when reading a uniformly divided straight or circular measurement scale. A vernier scale is a visual aid to take an accurate measurement reading between two graduation markings on a linear scale by using mechanical interpolation; thereby increasing resolution and reducing measurement uncertainty by using Vernier acuity to reduce human estimation error.