In 1839, Carlo Cattaneo published the first map illustrating the territorial distribution of the population in an Italian region (Lombardy). His publication was the first of a series of descriptive and illustrated records dealing with the numerical entity and the forms of population settlements. The aim of his survey was to clarify «the roots of the diversified density of our population». What results from the study of these settlements is that communities living side by side are actually more stimulated to develop than those in which the population is more scattered and widespread. But the greater the density of the population amassed in large agglomerations of buildings, the greater the need to rationalize territorial organization and to establish limits to offset the pressure exercised by this accumulation. For this purpose, a map illustrating population density is the first tool required when drafting any territorial project.