The ideal hydrogen-bonded contact of amide groups is postulated as a function of the geometry of the CONH2 bonds and of the lone pairs of the oxygen atoms. The observed packing arrangements of primary amides are analysed in terms of this ideal contact and of three elements of symmetry (glide plane; two-fold screw axis; translation) generating hydrogen-bonded networks from centrosymmetric amide pairs. The inter-relationship between molecular shape and conformation and packing modes is considered. The geometry of NH ⋯ O lonepair contacts and the NH ⋯ O distances in the observed packing types are also discussed.