There are several alternative treatments for the ceilings of arcades. They may be flat and panelled by beams carried across in a line with the pilasters and with a cornice moulding carried round the sides of the beams. [Illustration: No. 174. Doric Arcading.] The interior can also be vaulted by means of archivolts springing from the line of the imposts. The archivolts should be supported by pilasters at the back of the piers and on the opposite wall, and a cornice may be carried round between the vaults. Cross vaulting also may be employed, and in this case the entablature is no longer necessary. The proportions already given determine the width of piers when an order is used, but when an order is not used some further general rules for proportions are necessary. The height of the opening formed by arches, which may spring from piers--with or without an impost--should be about twice the width. The supporting piers should not be less than a third or more than two-thirds the width of the aperture. In any form of arcading, piers must be employed at the angles, and these should be wider than the intermediate ones by a half, a third, or a fourth. [Illustration: No. 175. Doric Arch, with pedestal.] _Subsidiary Order_ A secondary or subsidiary order is sometimes used in an arcading. The height of the arch should then be twice its width, and the height of the small order two-thirds the height of the column of the principal order. This height of two-thirds the column should be sub-divided into nine parts, of which eight will give the height of the column and the remaining one that of the entablature. The entablature consists of architrave and cornice, the frieze being omitted, and a division into five will give the relative proportions. Two-fifths may be taken as the height of the architrave and three that of the cornice. [Illustration: No. 176. Employment of the Subsidiary Order.] Pilasters are used with the columns of the subsidiary order with a space of half a diameter between them and the columns. [Illustration: No. 177. Subsidiary Order. Division of Entablature.] The archivolt should be equal in width to the upper diameter of the column, and the width of the lower edge of the keystone should also be of the same dimension. The subsidiary order may be the same as the principal order, but more often the Ionic is used in conjunction with the Doric, or the Corinthian with the Ionic. As regards the treatment of the bases, the horizontal alignment must be maintained. It is obvious that if the height of the base of the large column is adopted for the subsidiary one it will be very much out of proportion. This can be obviated by carrying through the plinth of the larger column to form a step on which the base of the smaller rests, and always the top line of the smaller bases should agree with that of the larger. _Superimposed Orders_