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.
The width of the die is the same as the plinth of the base of the column above it, that is, one and one-third diameter. The projection of the capping, which is the same for all three orders is obtained by drawing a line at an angle of 30 degrees with the vertical from the top of the die.
The width of the plinth corresponds to the projection of the capping, and is determined by dropping perpendiculars from the top fillet.
The height of the mouldings between the die and plinth is determined by a line drawn from the bottom of the die at an angle of 45 degrees to intersect the vertical face of the plinth. The angle is the same for all the orders.
Doric Pedestal
In the Doric order the height of the capping is one-third diameter. The face of the die is square, and this determines the height of the plinth.