The height of the window cells in such vaults was not at all fixed though it was quite frequently determined by the intersection of two ribs running diagonally from each side of the window to the second impost on the opposite wall of the church.[246] Such window cells as these naturally left a large central space along the crown of the vault, which was usually decorated by extra lierne and ridge ribs. TRACERY VAULTS Not content with the liernes as a decoration, an innovation appears in Tewkesbury choir,[247] Saint George’s Chapel at Windsor[248] and elsewhere, which consists in the application of raised mouldings forming tracery patterns on the few open spaces left between the ribs of complex lierne vaults. It is as if the tracery of a window were applied to a background of stone, with ribs taking the place of mullions. The patterns are usually trefoils or quatrefoils, but other forms, as, for example, the cross shaped flowers in the fan vaults at Peterborough (Fig. 39) also occur. The natural consequence of such added mouldings and ribs as those just described was to bring about the total sacrifice of the structural principles of ribbed vaulting to those which were purely decorative, and it is not surprising that such a vault as that of the choir of Wells cathedral (1329-1363),[249] in which the ribs have but the slightest claim to structural purpose should be found even at its early date as an example of this decadent stage in English vaulting. FAN VAULTING But the addition of multiple ribs lead not only to such debased vaulting as that at Wells. It must have played a large part in the creation of the distinctly novel construction known as fan vaulting. For in a vault with many tiercerons, as for example, that at Exeter (Fig. 37), or Hereford south transept,[250] the combined surfaces between the ribs is a cross between half of a hollow sided pyramid and a cone. This is true because, like most of the English churches, the wall rib is not highly stilted to concentrate pressures on a narrow strip of outer wall, or to leave a more pointed window head as in France, but it and the tiercerons and diagonals have much the same curvature. It was natural, therefore, that the English [Illustration: FIG. 39.--PETERBOROUGH, CATHEDRAL.]