The attainable precision in spectral classification in the MK system is studied making use of the recent "Catalogue of stars classified in the MK system" by Jaschek, Conde and Sierra. It is concluded that the internal dispersion of the different series is comparable and that it is of the order of half of a tenth in spectral class and half a luminosity class. The paper in full will be published in the proceedings of I.A.U. Symposium N " 24.