The sexpartite vaulting of the latter, together with that of the five other similarly vaulted Norman churches and of the choir of St-Denis at Paris, has always been an architectural puzzle, since it is manifestly a stage in the development of the oblong quadripartite vault, and yet is found in these cases some years after the latter system is known to have been fully understood in France, and nearly three-quarters of a century later than the vault of Sant' Ambrogio.