The same process may be applied to an area of any extent by repeating the ranges of piers, or limited to a single span or to a single compartment, at pleasure; and in all these cases it has the advantages of giving all the internal cubic space, and all the height of wall of which a vaulted area is capable; while, by concentrating the lateral pressure upon points at convenient intervals, where it may be readily resisted by external buttresses, it leaves the intervening wall-spaces at liberty to be pierced by windows, doors, or archways at pleasure. The Roman builders usually strengthened their vaulting by narrow strips of brickwork or cut stonework from pier to pier, constructing the rest of inferior materials. Their groined vaults were similarly fortified at the lines of intersection; but, as the whole was usually encrusted with plaster, these constructive expedients had no effect on the appearance. Sometimes, however, in their barrel vaults (as in the piscina at Baiæ, mentioned by Professor Willis, and in the corridors of the Colosseum), we find these strengthening strips appearing as ribs projecting downwards from the surface of the vaulting, and supported by projecting piers. The application of this to _groined_ vaulting is an obvious step, and adds vastly both to its strength and beauty. Let us suppose a length of vaulting so divided; we find at once that we are getting into a very sightly system, and one susceptible of excellent architectural treatment. Let us then, before proceeding to more advanced or intricate developments, apply to what we have reached the same process of architecturalisation which we have gone through for mere arching. Now, so far as relates to a barrel vault, it is evident that when divided by transverse ribs, those may be carried by pilasters or by colonnettes just as the orders of an ordinary arch (Fig. 317); and if we further mould or otherwise decorate the ribs and continue the capitals as an impost along the springing line, we have given a very fair amount of architectural character to the simplest form which vaulting can assume (Fig. 318). [Illustration: Fig. 317.] [Illustration: Fig. 318.] To pass on to the simplest form of _intersecting_ or _groined_ vaulting, it is equally clear that columns may be substituted for the square piers which are its normal supports. [Illustration: Fig. 319.--Crypt under the Church of the Holy Trinity at Caen.]