IIW-type (International Institute of Welding) ultrasonic calibration blocks are used widely throughout the world in nondestructive testing of materials and structures. They are used to establish certain physical characteristics of ultrasonic search units (transducers and plastic wedges) and flaw detection systems. Figures la and lb illustrate the geometries of the two popular U.S. variations of the IIW-type block: IIW-Type 1 design referenced by ASTM (American Society for Testing and Materials), and IIW-Type 2 commonly known as the U.S. Air Force design. The blocks are nominally 300 mm (12 in.) long, 100 mm (4 in.) wide, and 25 mm (1 in.) thick. The geometry, physical characteristics, and uses of the ASTM design are specified in the ASTM standard E-164, while the Air Force design is described in the U.S. Air Force Technical Manual on Nondestructive Inspection Methods. A comprehensive summary of the different block designs and physical characteristics is given by Hotchkiss [1].