This document specifies micrographic methods of determining apparent ferritic or austenitic grain
size in steels. It describes the methods of revealing grain boundaries and of estimating the mean grain
size of specimens with unimodal size distribution. Although grains are three-dimensional in shape,
the metallographic sectioning plane can cut through a grain at any point from a grain corner, to the
maximum diameter of the grain, thus producing a range of apparent grain sizes on the two-dimensional
plane, even in a sample with a perfectly consistent grain size.