A _bay_ of a cathedral is one of the spaces into which its length is divided by the supports of the roof as piers, arches, or principals. The bays of the aisles are usually square--those of the nave, choir, or transepts rectangular on account of their greater width. Vertically each of the bays of the nave, etc., is divided into three stories (Plate XI., Fig. 2), the _groundstory_ rising from the floor; the _triforium_, or _blindstory_, having no windows, is over the aisles, and the _clerestory_ over the triforium. The prefix _clere_--bright--indicates the brilliancy of its light. In the Norman period these three divisions were nearly equal in height. In the succeeding periods the groundstory attained about half of the total height of the bay, the clerestory was extended downwards, and the triforium reduced, until, in the Perpendicular period, it entirely disappeared. Plate XI., Figs. 3 and 4, show a few square bays of Gothic vaulting in skeleton diagrams with the forms of plan indicated by dotted lines upon their base-planes. All the lines represent ribs. Plate XI., Figs. 2 and 3, show _quadripartite_ vaulting--_i.e._, having four compartments in one bay. This is the simplest form of Gothic vault, and belongs chiefly to the Early English period. The ribs ah, bh, ci, di are wall ribs; bg, cg, ag, dg, are cross ribs; ae, ce, be, de, are the diagonal ribs. The ridge-ribs eg and h, e, i, are horizontal, and intersect the summits of the cross ribs and diagonals. At every intersection there is generally a carved keystone or _boss_. In the vaulting of a nave the breadth across is about twice the breadth of the aisles, so that the nave bays are not square, but rectangular. Plate XI., Fig. 4, and Plate XII., Fig. 2, show _lierne_-vaulting, having _lierne-ribs_, the short ribs joining and supporting all the ascending ribs as h, k, l, m, g. Plate XII., Figs. 2 and 3, are lierne vaults. That shown in Fig. 3 is under the belfry of a church tower at Coventry, with circular opening for hoisting the bells. The detail at B shows the method of collecting the three ribs into one at the _springers_ in the corners A, B, C, D, by small arches in the tracery of ribs. Plate XII., Fig. 1, shows fan-vaulting having no ribs. The lines shown indicate mouldings on the masonry imitating ribs. The structure is built up of slabs of stone, accurately joined together forming concave half-cones, their vertices being the springers of the vault. The dotted lines show some of the jointing; the other lines represent the imitation ribs. The crown of the vault is the flat surface gh, gi, generally richly ornamented. INDEX Abacus, Pl. III., Fig. 1 Arcade (Norman), Pl. III., Fig. 7 Ball-Flower, Pl. VI., Fig. 3 Band, Pl. V., Figs. 10, 11 Base, Pl. III., Figs. 1, 5 Battlements, Pl. IX., Figs. 4A, 4B Bay, Pl. XI., Fig. 2 Belfry Windows, Pl. IX., Figs. 1, 2 Boss, Pl. XII., Figs. 2, 3 Braces, Pl. X., Figs. 2, 7 Brattishing, Pl. VIII., Fig. 5