This Standard descrives the material, dimensions, manufacture and method of use of three calibration blocks. The first one is designed solely for use with flaw detectors employing either one or two shear wave probes, while the second, of the type recommended by the International Institute of Welding, and the third, developed in Switzerland, are designed for a wider range of calibration measurements. It should be noted that the blocks described cannot be used as a substitute for full laboratory calibration of the electrical and acoustical variables involved and cannot be employed for estimating the sixe of an imperfection. When using such blocks for calibration purposes, it is emphasized that all adjustments and measurements should be made with the same flw detection apparatus, attenuators, impedence matching devices, proves, probe leads, etc., which are to be used for inspection purposes, and the frequencies appropriate to the work to be examined.