In earlier work (1–3) we tried to establish correlations between molecular size or shape and crystal structure descriptors for organic crystals containing C,H,N and O atoms, without fully developed hydrogen bonds. Samples of 391 hydrocarbon, 590 oxohydrocarbon, and 458 azahydrocarbon crystal structures, retrieved from the Cambridge Structural Database (4), were used. Of course size and shape alone are important for crystals which are built under the action of purely isotropic, or at least scarcely directional, intermolecular forces; ideally, for non polar substances like hydrocarbons. In fact, in crystals containing O or N atoms, a rather large number of C-H… X (X=O or N) attractive interactions were detected through a search of the spurious repulsions that result from their treatment as pure van der Waals contacts. These interactions are now recognized as weak hydrogen bonds, and also display some angular selectivity: Figure 1 shows that C=O… H angles for carbonyl compounds cluster in the 110–140° range for R(O… H) < 3Å.