These indicators come with a calibration certificate traceable to NIST that states they’ve passed a test for accuracy. They have a balanced dial with positive values on one side of the zero and negative values on the other side for comparative measurements of a part's tolerance against a reference measurement. A spring-loaded plunger retracts and extends to measure objects. All have highly sensitive, wear-resistant jeweled bearings for a long service life. Rotate the dial face for viewing at multiple angles.