It was simply as follows:--First imagine, for argument’s sake, that the architect had intended to divide his space into three spans--a wide and two narrow ones--like the Lady Chapel at Salisbury, or the crypt of the Sainte Chapelle at Paris, supporting the vaulting on ranges of thin pillars (Fig. 395). The setting out of the divisions was about the following. The whole span being divided into five parts, two were given to the width of a bay, one to the imaginary aisle, and three to the central span. The vaulting of the aisles, then, would be in oblongs of double their width _set lengthway_ of the building, and the central span in oblongs half as long again as their width _set crossway_ to the building. [Illustration: Fig. 395.] Let us apply to each the rule I have already laid down for the fan vaulting of oblongs. Beginning with the ideal aisles, from the pillars and responds and centres draw the lower circles of the fans reaching the apex of the _narrower_ arches, and the upper circles reaching the apex of the _wider_ arch (Fig. 396). This gives us an oblong fan vault in its most normal form. [Illustration: Fig. 396.] Then do precisely the same with the wider or central span, and our space is covered with oblong fan vaulting of the most usual kind. Now, as the lower arch of the central span is identical with the higher arch of the side spans, it follows that the fans in the sides (or aisles) are continued and completed in the central vault, the lower arch of the small vault being also continued round to complete the design. The decoration of the smaller fans then is in _two_ stages, and that of the larger ones in _three_; but, the design of both being continuous, the one is only an extension of the other. Had the architect stopped here, no system of vaulting, on the fan principle, or space so divided, could be more systematic or more simple in its ideal. He had no thought, however, of stopping at so commonplace a stage, and his pillars were designed only to do their work on paper, and then to be erased. The columns were omitted, and their places supplied by pendants; but, as such a roof could not stand for a moment, something must be done to supply the support which the pillars would have afforded. [Illustration: Fig. 397.--Henry VII’s Chapel, Westminster Abbey.]