A series of N-phenyl-2-hydroxynicotinanilides (PHNAs) have been synthesized, and their crystal structures have been investigated.The identity of these compounds is revealed to be the tautomer of PHNAs, i.e., 2-oxo-1,2-dihydropyridine-3-carboxylic acid phenylamides, compounds with both amide and lactam functionalities.Among these compounds, those with no or negligible steric hindrance take a near planar conformation and form lactam-lactam dimers, not the widely observed amide NH···O=C hydrogen bond chains, while those with steric hindrance take a twisted conformation and form an amide NH···O=C (lactam) hydrogen bond.A succinct theoretic study combining conformational anal. and mol. electrostatic potential surfaces (MEPS) survey provided a reasonable explanation of the formation of these two different hydrogen bond motifs.The identity of N-phenyl-2-hydroxynicotinanilides is revealed to be their tautomers, 2-oxo-1,2-dihydropyridine-3-carboxylic acid phenylamides, compounds with both amide and lactam functionalities.The steric effect plays an important role in the formation of lactam-lactam dimers or amide-lactam catemer motifs in these compoundsTheor. studies have provided a reasonable explanation for the exptl. observations.