Ab initio quantum calculations are applied to both the parallel and the antiparallel arrangements of the β-sheets of proteins. The energies of the NH···O and CH···O hydrogen bonds present in the β-sheet are evaluated separately from one another by appropriate modifications of the model systems. The bond energies of these two sorts of hydrogen bonds are found to be very nearly equal in the parallel β-sheet. The NH···O bonds are stronger than CH···O in the antiparallel geometry but only by a relatively small margin. Moreover, the former NH···O bonds are weakened when placed next to one another, as occurs in the antiparallel β-sheet. As a result, there is little energetic distinction between the NH···O and CH···O bonds in the full antiparallel β-sheet, just as in the parallel structure.