The width of the plinth is one and a third diameter. The height of half a diameter does not include the top fillet. The proportions may be approximated by dividing the height into four. One of these parts will be the height of the plinth, another that of the large torus and its fillet, the third--the upper edge of fillet of top scotia, and the fourth that of the upper scotia and torus with the intervening fillet. Vertically the uppermost fillet is in alignment with that of the upper scotia, and the extremity of the upper torus with the small bead mouldings dividing this from the lower scotia. Although this is the orthodox Corinthian base, it is not used as frequently as the Ionic type, but when the order is on a large scale the more elaborate version is justified. _The Arch_ When the arch is used in conjunction with the column it is supported on pilasters which are attached to the columns. The columns and pilasters thus form piers. In the Doric order the columns are placed five diameters apart from centre to centre, in the Ionic five and a quarter, and in the Corinthian five and a half. The necking moulding of the capital is generally carried through above the arch, the outer line of which is struck so as to nearly touch the underside. The projection of the pilasters which carry the arch is half a diameter from the outer lines of the column, and is measured on the same level as the springing point of the arch. The centre from which the arch is struck is sometimes in the same horizontal line as the springing points, but more frequently a little above the line and thereby a rather better effect is produced. The arch-band or archivolt is the same width as the pilaster supporting it, and a series of mouldings known as the impost is placed at the top of the pilaster. The base of the pilaster consists of a plain plinth of slight projection equal in height to the base of the column. From the spacing of the columns and the proportions here given it will be seen that the height and width of the aperture made by the arch and pilasters are arrived at automatically, but if measured, the height will be found to be about twice the width and the top of the impost about two-thirds the height of the column. These proportions may be accepted as giving satisfactory results under ordinary conditions. As a general rule, in all the orders the impost is half a diameter high, and so is the same as the projection of the pilaster and the width of the archivolt. This rule is not always adhered to, however, but in any case the width of the archivolt should never be more than one-eighth or less than one-tenth of the diameter of the arch, and should always be the same as the width of the pilaster. _Doric Impost_ The mouldings of the Doric Impost are as follows: