ISO 18653:2003 specifies methods for the evaluation of measuring instruments used for gear measurements of involute, helix, pitch and runout. It is applicable both to instruments that measure runout directly and to those that compute it from index measurements. It also gives recommendations for the evaluation of tooth thickness measuring instruments and, of necessity, includes the estimation of measurement uncertainty with the use of calibrated gear artifacts. It does not address the calibration of artifacts by laboratories accredited in accordance with ISO 17025; nor are its requirements intended as an acceptance specification of product gears (see ISO 1328-1, ISO 1328-2, ISO/TR 10064-1 and ISO/TR 10064-2). The estimation of product gear measurement uncertainty is beyond its scope (see ISO/TR 10064-5 for recommendations).