Abstract
Even if we confine ourselves to the classical HR diagram, postponing the transformation of spectral types and visual absolute magnitudes into basic physical variables, there are several ways to choose the data. If, for example, we want the density of points to represent the spatial density of stars, then we must concentrate on reducing sampling errors, usually at the cost of accuracy in some parts of the diagram. The Hess diagram is very useful, but an equally useful alternative is an HR diagram which includes only those stars whose positions are determined as accurately as possible. The objective is to classify the fine structure in the pattern in the luminosity-spectrum domain, and thus to define more precisely the borders of zones which are either occupied or avoided by stars of a given population. It is this pattern which is compared with theoretical evolutionary tracks and it is such a diagram based on the revised MK classification of the recent Atlas of Keenan and McNeil (1976) that I shall present here.
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