Latin abaque, or abacus, abaco,
The upper member, or division, of a capital, on which the entablature, in Classic architecture, rests. It forms an essential part of the column in the Grecian and Roman styles, and is found in almost every variety of column in Christian architecture. Its shape varies in all the classic orders. In those of the Tuscan and Doric, the abacus is plain, thick, and rectangular in plan; but in the Corinthian, Roman-Ionic, and Composite, its sides are ornamented, and, in the latter, are cut into eccentric cavities, or curves, each of which is generally adorned with a flower or other enrichment. In general practice, the angles of the abacus are cut off in segments of circles. In the Temple of Jupiter Olympus, at Athens, the angles terminate in sharp points.
In Christian architecture, the abaci form the bases of arches and in shape and ornament is greatly diversified, as exemplified in the accompanying plates of Capitals and Columns. The square abacus may be a guide to distinguish the Norman from " the early English,"; but several examples of Norman buildings may be pointed out where the abaci are circular and octangular. These eccentricities shew that we must not attempt to include all the varieties within a few systematic forms. In all the members of the column and arch, as in their sizes, proportions, and shapes, the monastic architects were unrestrained in their designs, and unfettered by arbitrary rules. Though not an invariable, it may be regarded as a general maxim in Christian architecture, that the abaci as well as the capitals were in the earliest examples square, next circular, then octagonal: they often varied from the form of the capital.