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.
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.
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.