Cyclopedia of Biblical, Theological and Ecclesiastical Literature
(Lat. from ἄβαξ , a board). This name is applied in architecture to the uppermost member or division of a capital.
1. In the Grecian and Roman orders it is a very essential feature.
In the Grecian Doric the abacus has simply the form of a square tile without either chamfer or moulding.
In the Roman Doric it has the addition of an ogee and fillet round the upper edge.
In the Tuscan a plain fillet with a simple cavetto under it is used instead of the ogee and fillet. In all these orders the abacus is of considerable thickness, and the moulding round the upper edge is called the cimatium of the abacus.
In the Grecian Ionic it is worked very much thinner, consisting of an ovolo or ogee, generally without any fillet above it, and is sometimes sculptured.
In the Roman Ionic it consists of an ogee or ovolo with a fillet above it.
In all the preceding orders the abacus is worked square, but in the modern Ionic, the Corinthian, and the Composite, the sides are hollowed, and the angles, with some few exceptions in the Corinthian order, truncated. The mouldings used on the modern Ionic vary, but an ogee and fillet like the Roman are the most common. In the Corinthian and Composite orders the mouldings consist of an ovolo on the upper edge, with a fillet and cavetto beneath.
2. In the architecture of the Middle Ages, the abacus still remains an important feature, although its form and proportions are not regulated by the same arbitrary laws as in the classical orders: in the earlier styles there is almost invariabily clear line of separation to mark the abacus as a distinct division of the capital; but as Gothic architecture advanced with its accompanying variety of mouldings, the abacus was subject to the same capricious changes as all the other features of the successive styles, and there is often no really distinguishable line of separation between it and the rest of the capital.
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McClintock, John. Strong, James. Entry for 'Abacus'. Cyclopedia of Biblical, Theological and Ecclesiastical Literature. https://www.studylight.org/encyclopedias/eng/tce/a/abacus.html. Harper & Brothers. New York. 1870.