This classification for grain size comprises three sets of comparison charts to be used for determining grain size. These charts are presented in three categories as follows: Plate I - Untwinned grains (flat etch) Plate II - Twinned grains (flat etch) Plate IV - Austenite grains in steel (McQuaid-Ehn test or other test) Table 1 lists a number of materials and the comparison charts that are suggested for use in estimating their grain size by the comparison method. NOTEÂ- The suggestions in Table 1 are based upon the customary practices in industry. For specimens prepared according to special techniques, the appropriate comparison chart should be selected on a structural appearance basis as described in the Scope.