The instrument to be used to obtain the linear dimension of an object will depend on its contour and/or location and the degree of accuracy desired. Rules, scales, tapes, calipers, depth gages, etc, are used for such measurements. Some of them are direct measuring instruments and others are comparison instruments which must be referred to appropriate standards or direct measuring instruments. The linear dimensions of practically any object are subject to variation depending on the temperature of the object. All standards of length are calibrated at 68F (20C) and all measuring instruments are intended to be correct within manufacturing limits at that temperature. At any other temperature they are necessarily some other size, but if the measuring instrument and object to be measured are of the same material they will change equal amounts at temperatures other than 68F and no error will be introduced. For dissimilar materials only the difference in expansion or contraction between the measuring instrument and the measured object need be taken into account.