Whatever the cause of its introduction into the Ile-de-France may have been, the six-part system was used at Saint Denis (Seine) (1140-1144) and soon became the favorite method throughout the neighboring region. Unfortunately Saint Denis and two other important churches of the Transition, the cathedrals of Senlis (Oise) (cir. 1150) and Noyon (Oise) (cir. 1140), which would undoubtedly have illustrated the progress in six-part vaulting, no longer have their original vaults, and the cathedral of Sens (Yonne) (1140-1168) (Fig. 28) remains as perhaps the most important example of the early developed type.[188] Its vaults show the great advance made in construction since the completion of Saint Étienne at Caen. The diagonals are semicircular instead of segmental arches, and the transverse ribs are pointed and all of similar curve, giving a more symmetrical appearance and greatly reducing the thrusts. Furthermore the piers are profiled from the ground according to the load which they are to carry, and, last of all, a highly stilted wall rib is added over each clerestory window, completing the skeleton of the vault and making possible a larger expanse of glass and more satisfactory illumination for the interior. Of course, the use of the flying buttress, which had been introduced a short time before Sens was built, contributed enormously to the advancement of vault construction and in large measure explains such an improved form of vaulting as this is. In fact, a heavy clerestory wall was no longer essential to the support of the vault and it was only the fact that a large expanse of glass was not safe from the pressure of the wind, which prevented the clerestory windows from occupying the entire [Illustration: FIG. 28.--SENS, CATHEDRAL.]