Abacus (architecture)

In architecture, an abacus (from the Greek abax, slab; or French abaque, tailloir; plural abacuses or abaci) is a flat slab forming the uppermost member or division of the capital of a column, above the bell. Its chief function is to provide a large supporting surface, tending to be wider than the capital, to receive the weight of the arch or the architrave above. The diminutive of abacus, abaculus, is used to describe small mosaic tiles, also called abaciscus or tessera, used to create ornamental floors with detailed patterns of chequers or squares in a tessellated pavement.

In classical architecture, the shape of the abacus and its edge profile varies in the different classical orders. In the Greek Doric order, the abacus is a plain square slab without mouldings, supported on an echinus. In the Roman and Renaissance Doric orders, it is crowned by a moulding (known as "crown moulding"). In the Tuscan and Roman Doric capital, it may rest on a boltel.

In the archaic Greek Ionic order, the abacus is rectangular in plan, owing to the greater width of the capital, and consists of a carved ovolo moulding. In later examples, the slab is thinner and the abacus remains square, except where there are angled volutes, where the slab is slightly curved. In the Roman and Renaissance Ionic capital, the abacus is square with a fillet on the top of an ogee moulding with curved edges over angled volutes.

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