When the ribs became more numerous, it often occurred that five or even eight of them had to spring from one group of capitals; and at times three at least (and subsequently more) from a _single_ capital. It is clear that in such cases the three or more distinct forms could scarcely retain their separate existence, but that being united in their lower portions in a single block, their forms would, more or less, die one into another. Though we can trace this process in Norman work, it was not completely established till some time later. [Illustration: Fig. 358.] [Illustration: Fig. 359.] It will be better understood by means of Fig. 358, in which a transverse rib, two diagonal ribs, and two wall ribs meet at their common springing line, and so intersect and unite one with another as to produce a section at the base composed of portions of them all. Now, a drawing of this group of ribs will at once show that their combined and united form must extend to some considerable height above the springing; and so far as it reaches, which is often some 8 feet or 10 feet in height, they cannot possess an individual existence (Fig. 359). To this height, then, it is customary to build the group of ribs in horizontal courses, and only to commence the radiating arch-joints where the ribs clear themselves one from another, which usually occurs at one level, though in vaults of great irregularity one rib often clears itself at a lower level than another. In setting out the relative position of the ribs upon the common springing level, great skill and judgment are requisite, or they will clear themselves one of another so irregularly as to cause great difficulty and needless twisting in the filling in of the vaulting surface. If you set out on plan the side lines of two ribs (Fig. 360), and lay down the true position of the mouldings of _one_ of them, it is clear that, if the curvature [Illustration: Fig. 362. Fig. 360. Fig. 361.]