Glass plates of the PM type are designed to check the flatness and mutual parallelism of the surfaces of micrometers and staples by the interference method. Plane-parallel glass plates have the form of round straight cylinders with polished end surfaces, which are measured. To test tools with different measurement limits, plates are manufactured in four sizes: PM15, PM40, PM65, PM90. Plates come in sets of 4. Plate material - K8 glass