During the short introductory stage before 1150 the cupola had the upper hand and imposed its construction on the intersecting ribs just imported from the north. The earliest _bombé_ vaults with ribs are really cupolas still, since the stones of their infilling were laid in concentric rings round and round. Only a small number of these ribbed cupolas were built. Then in the first phase of Plantagenet Gothic appeared the ascendency of ribbed vault over cupola. The dome was lowered and the stones of the infilling were laid like those of a true Gothic vault, not horizontally, round and round, but vertically, with the courses running parallel with the ridges of the triangular compartments traced by the diagonals. Each of the four triangular cells was concave in both directions, with a groin defining its axial line. Hence eight panels, not four, composed the _bombé_ vault, groin ridges alternating with ribs. Such groin lines called for strengthening ribs beneath them, since a curving surface has more need of a bone skeleton to stiffen it. Given the _bombé_ shape, it was inevitable for the architect to arrive soon at the use of ridge ribs between the diagonals. The Plantagenet vault _par excellence_ is made up of eight ribs that branch from a central keystone, those ribs being of the same slight graceful profile as the arches framing each vault section. For a time the rib molds of the First Period were enormously heavy and wide, like the diagonals of the nave of Angers Cathedral--the oldest Angevin Gothic work extant (c. 1150). Their profile shows two large round molds with a flat space between. Before long the level space tended to swell into a roll molding, which in time predominated over the lateral ones; such are the diagonals of the Trinité church at Angers (c. 1170). Finally, the side rolls died out altogether, leaving one slender uniform torus, a characteristic of the Second Period of Plantagenet art. When the lateral and transverse arches adopted the same delicate profile as that of the eight branching ribs, there was achieved the slender elegance and rare distinction typical of the best Plantagenet interiors. Keystones were richly carved, and pretty figures and heads were added where the vault ribs met the framing arches. During the last quarter of the XII century the Plantagenet school was building vaults of this type, and they remained in vogue till the cuplike shape died out altogether. In Plantagenet art the ramification and intercrossing of ribs had a structural reason, since they were the logical result of the concave outline of the vault and not, like the supplementary ribs of Flamboyant Gothic, mere ornamentations.