A combined geometric/chemical model is proposed to account for the observed lattice dimensions of the cubic AB2 Laves phases. These compounds are treated as a hybrid structure between the simple metallic bcc and fcc structures, the interatomic spacings depending on the atomic diameters of the pure A and B metals and on the difference in their electronegativities. The model is successful enough to indicate that the electronegativity concept could well find wider quantitative applications in alloy chemistry than it has in the past. The treatment used also leads to qualitative explanations for the relative frequency of occurrence of cubic and hexagonal Laves phases, the incidence of allotropy in these compounds, the nonappearance of Laves phases in certain apparently favorable alloy systems, and the minimum A-metal content found in phases showing a range of homogeneity.