The accessory comprises a measuring standard having first and second spaced faces, the faces a known distance apart. The measuring standard has a uniform external surface in a direction at right angles to the spaced faces. A sleeve is provided which has a surface sized to engage upon the external surface of the measuring standard so that the standard is slidable with respect to the sleeve in a direction at a right angle to the faces so that the measuring standard can engage against a measuring surface of a micrometer or caliper by sliding of the standard in the sleeve to reduce the measuring gap by a distance equal to the length of the standard and position the measuring gap that distance away from the measuring surface whereby the standard can be inserted in the sleeve at the location where the measurement is to be made without adjustment so that accurate adjustments can be made at remote locations away from measuring references. The measuring standard is a right circular cylinder
