From the photoelectric system of Geneva Observatory, we establish a three-dimensional representation for the stars of spectral type between A₀ and G₅ and of class of luminosity V and IV. The effects of blanketing can thus be made conspicuous and calculated for the indexes of colour and the parameter that we use.

The application of this method to the Am stars shows that these last ones are included, in a Mν/B₂-V₁ diagram, in a zone of one magnitude, situated above a sequence of reference, sequence the most possible approaching a ZAMS. Far more, it is a correlation between the metallic character and the velocity of rotation of the star. ln this representation, the subdwarfs are situated among the dwarfs deficient in metals, that is to say with a low value of [Fe/H]. It is also possible to distinguish giant stars of population II and stars of horizontal branch in the H-R diagram.