ABACUS, in architecture, signifies the superior part or member of the capital of a column, and serves as a kind of crowning to both. It was originally intended to represent a square tile covering a basket. The form of the abacus is not the same in all orders: in the Tuscan, Doric, and Ionic, it is generally square; but in the Corinthian and Composite, its four sides are arched inwards, and embellished in the middle with
some ornament, as a rose or other flower. Scamozzi uses abacus for a concave moulding on the capital of the Tuscan pedestal; and Palladio calls the plinth above the echinus, or boultin, in the Tuscan and Doric orders, by the same name. See plate I. fig. 1. and ARCHITECTURE.