In the Romanesque and Gothic styles, the vault is composed a series of modules, called bays. In the quadripartite vault pictured here, the bay, contained within the space of three columns formed by three transverse arches which bridge the gap between facing bundles of columns. The space between the two outside arches is articulated by diagonal arches which form, with the central transverse arch, a six-armed spider. The place where diagonals and tranverse arch meet forms the keystone, a crucial structural element in the support of the stone canopy.

The construction by repetition of identical modules, which is at the heart of the monumental architecture of the 12th and 13th centuries, is an innovation of Romanesque style. It allows the observer to grasp immediately both the unity of the enclosed space and the logical units of which it is composed.