A little later the colonnettes themselves become grouped in threes and fours, and their edges often filleted, or “keeled,” that is, decorated by an arris or edge projecting from their round surface. Thus, at Lichfield (Fig. 307) in the older portions, groups of three shafts united into one, and carrying a common abacus, were set on each side of an octagonal bearing-shaft. At Wells (Fig. 308) similar triple shafts were set alternately against the faces and in the internal angles of a cross-formed nucleus, with alternately square and octagonal abaci. [Illustration: Fig. 307.--Lichfield.] [Illustration: Fig. 308.--Wells.] My purpose, however, is not to enumerate all possible varieties of clustered pier, but to explain its principle, and at the same time to show how unlimited an artistic element was deducible from an intent thus founded on the natural conditions of arched construction. To go much farther would carry us on prematurely into the succeeding styles, and would be also anticipating another cause, which carried on the principle to a still further development. I allude to groined vaulting, of which I shall have to treat in detail when I reach it. Before, however, I quit the subject of arches and piers, I must say a few words on the application of their principles to doorways and windows. Doorways differ in no degree, as to principle, from archways, excepting in having, at some point in the thickness of the wall, more or less recessed at pleasure, what Professor Willis calls the “doorway plane;” that is to say, one of the arch-orders so formed that the door may be hinged to it, and may shut against it. The actual opening of the door may or may not be stopped on this plane to a square heading, the arch over it being filled in with a tympanum, plain or sculptured; or it may be altered from the form of the main arch to some shape having less height. In all other respects the principles already stated apply equally to doorways as to archways. The interior, however, has to be varied if the door fills in the arch-form, with a view to facilitating its free opening; but this is a practical point not needful to be here gone into. The orders of arch-mouldings in a doorway often continue down the jambs, as in one of the magnificent doorways at Malmesbury Abbey: or they may be replaced by colonnettes or pilasters, or these methods may be united in the same doorway,--just as in another door at Malmesbury, continuous mouldings alternate with colonnettes,[36]--and the arches, jambs, and capitals, and even the shafts of the colonnettes, may receive any degree of sculptured enrichment. The doorway being a point on which much architectural character was concentrated, and great depth being necessary to give the required effect, it was customary to thicken the walls at the doorways by various expedients, so as to obtain depth enough to give several orders of arch-mouldings; this increased thickness was covered over by gables, and by other means.