This Part of EN 623 describes manual methods of making measurements for the determination of mean linear intercept grain size of advanced technical ceramics using photomicrographs of polished and etched test pieces. This is not the true mean grain diameter, but a somewhat smaller parameter representing the average path length of a line drawn across a two-dimensional section. The relationship to true grain dimensions depends on grain shape and degree of microstructural anisotropy. This standard contains two methods, A and B. Method A applies to single-phase ceramics, and to ceramics with a principal crystalline phase and a glassy grain-boundary phase of less than about 5% by volume for which intercept counting suffices. Method B applies to ceramics with more than about 5% by volume of pores or secondary phases, or ceramics with more than one major crystalline phase where individual intercept lengths are measured, which can optionally be used to create a size distribution. This latter method allows the pores or phases to be distinguished and the mean linear intercept size for each to be calculated separately. NOTE A method of determining volume fraction(s) of secondary phase(s) is under development as ENV 623-5; this will provide a means of determining whether Method A or Method B should be applied in borderline cases. Some users of this standard may wish to apply automatic or semiautomatic image analysis to micrographs or directly captured microstructural images This is permitted by this standard provided that the technique employed simulates the manual method (see clause 4 and 8.4).