At the top there is a fillet and a bold ovolo, below which there is a bead-moulding or small torus with fillet and two facias. The lower facia, which is of slight projection is one-fourth of the total height. The upper facia and fillet are half the remainder. [Illustration: No. 168. Detail of Archivolts and Imposts.] _Archivolt_ The archivolt mouldings are in the Doric order, the same as those of the impost. _Ionic Impost_ The total height of half a diameter may be divided into two, and the upper half sub-divided into three. Then the upper division will be the height of the top fillet and a reversa moulding; the second the height of a facia and the third an ovolo and small torus. The lower half of the impost consists of two facias, the upper of which is broader than the lower. _Ionic Archivolt_ The archivolt, commencing at the outer rim, consists of a fillet and bold reversa moulding which occupies rather less than a fourth of the total width, and two facias of unequal widths. The width of the inner facia is nearly one-third more than that of the reversa moulding. _Corinthian Impost_ The Corinthian impost differs from those of the other two orders in having a necking and necking moulding in place of the two unequal facias. The total height should be divided into two, and the upper half divided into four. Then the top fillet and reversa moulding will be one of these divisions, the facia two, and the supporting ovolo will occupy the remaining one. The lower half of the impost consists of a small torus and fillet beneath the ovolo, the necking, and the torus and fillet which form the necking moulding. _Corinthian Archivolt_ The archivolt, commencing at the outer rim, consists of a fillet and reversa and three unequal facias. If the total width is divided into two, the point of bisection will be the centre of a small reversa moulding between the two outer facias. The outer reversa and the inner facia are each about one-sixth of the total width and the small reversa is about two-thirds of the inner facia. _The Keystone_ A projecting block, or keystone, is sometimes used at the centre of the arch. The face width of its lowest edge should not be less than the width of the archivolt. Its height is not often less than one and a half times or more than twice this width. It may be decorated in various ways, and is frequently in the shape of a console. This form is especially suitable when the keystone comes in contact with the architrave of the entablature, in which case it is capped with a moulded abacus. _The Pedestal_ The height of the pedestal is a fourth that of the column and entablature together, though this proportion may be varied to suit different conditions. For instance, when the pedestal forms part of the structure of a balcony or balustrading the height must be modified to suit the special requirements of the position. The pedestal is composed of plinth, die and capping.