It may be useful here to recapitulate what has been said of the subdivision of churches into bays, or, as the French call them, _travées_. The two typical arrangements of these are shown in Woodcuts Nos. 616 and 617, as existing before the introduction of the pointed forms. In the first a great gallery runs over the whole of the side aisle, introduced partly as a constructive expedient to serve the purpose for which flying buttresses were afterwards employed, partly as enabling the architect to obtain the required elevation without extraordinarily tall pillars or wide pier-spaces, both which were beyond the constructive powers of the earlier builders. These galleries were also useful as adding to the accommodation of the church, as people were able thence to see the ceremonies performed below, and to hear the mass and music as well as from the floor of the church. These advantages were counterbalanced by the greater dignity and architectural beauty of the second arrangement (Woodcut No. 617) where the whole height was divided into that of the side-aisles and of a clerestory, separated from one another by a triforium gallery, which represented in fact the depth of the wooden roof requisite to cover the side-aisles. When once this simple and beautiful arrangement was adopted, it continued with very little variation throughout the Middle Ages.[43] The proportions generally used were to make the aisles half the height of the nave. In other words, the string-course below the triforium divided the height into two equal parts; the space above that was divided into three, of which two were allotted to the clerestory, and one to the triforium.[44] It is true there is perhaps no single instance in which the proportions here given are exactly preserved, but they sufficiently represent the general division of the parts, from which the architects only deviated slightly, sometimes on one side, sometimes on the other, according to their taste or caprice. The only really important change afterwards introduced was that of glazing the triforium gallery also, by adopting a flat roof, or one nearly so, over the side-aisles, as the nave in the church of St. Ouen at Rouen, or by covering each bay by a pyramidal roof not seen from the interior, as is shown in the Woodcuts Nos. 621 and 641; the whole walls of the church, with the slight exception of the spandrils of the great pier-arches, having thus become walls of glass, the mass of the vault being supported only by the deep and bold constructive lines of which the framework of the glazed surfaces consists.