Abacus

In architecture, a crowning rectangular block or topmost stone on the cap of a pillar or column capital, providing support to an architrave or arch.

Abacus. (Arch.) A tablet placed upon the capital of a column, which adds to the surface of the capital and so enables it the better to support the superincumbent architrave. Some monuments are found in Egypt in which the capital is nothing but an abacus. Other monuments, however, present beneath the abacus a capital consisting of lotus flowers or acanthus leaves. In Greek and Roman architecture the abacus varies according to the order. In the Doric order the abacus is strong and simple in outline, while in the Ionic it is ornamented, and in the Corinthian it is curvilinear instead of straight. In the Gothic style the abacus varies according to the period. In the Romanesque period, i.e. in the 12th and 13th centuries, it is square, and projects beyond the foliage of the capital; in the 13th century it is sometimes polygonal, and very often the foliage of the capital projects beyond it. There are also found, chiefly in Normandy, some abaci belonging to this period perfectly circular in shape. In the 14th century they project less, and in the 15th their importance still more diminishes. At the Renaissance the ancient orders were restored to honour, with some modifications, and the abacus then regained the dimensions which belonged to it in the Greek and Roman orders.

A calculating table or frame, specifically one in which balls slide upon wires, used for the mechanical solution of arithmetical problems.