In architecture, an abacus (from the Greek abax, a slab; or French abaque, tailloir ) is a flat slab or shelf that sits upon the capital of a column, forming its uppermost member. Its chief function is to provide a large supporting surface (wider than the capital) to receive the weight of the arch or the architrave above.

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. In the Roman and Renaissance Doric orders, it is crowned by a moulding (a so-called "crown molding").

In the Archaic Greek Ionic order, owing to the greater width of the capital, the abacus is rectangular in plan, 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.

In the Greek Corinthian order, the abacus is moulded, its sides are concave and its angles canted (except in one or two exceptional Greek capitals, where it is brought to a sharp angle); and the same shape is adopted in the Roman and Renaissance Corinthian and Composite capitals, in some cases with the carved ovolo moulding.

In Romanesque architecture the abacus survives as a heavier slab, generally moulded and decorated. It is often square with the lower edge splayed off and moulded or carved, and the same was retained in France during the medieval period. In Early English work, a circular deeply moulded abacus was introduced, which in the 14th and 15th centuries was transformed into an octagonal one.

In Gothic architecture the moulded forms of the abacus vary; it may even be a flat disk or drum. The form of the Gothic abacus is often affected by the shape of a vault that springs from the column, in which case it's called an impost block.

The diminutive of abacus, abaciscus, is applied in architecture to the chequers or squares of a tessellated pavement.

Examples in England

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Early Saxon abaci are frequently simply chamfered, but sometimes grooved as in the crypt at Repton (fig. 1) and in the arcade of the refectory at Westminster Abbey. The abacus in Norman work is square where the columns are small; but on larger piers it is sometimes octagonal, as at Waltham Abbey. The square of the abacus is often sculptured with ornaments, as at the White Tower and at Alton, Hampshire (fig. 2). In Early English work the abacus is generally circular, and in larger work, a group of circles (fig. 4), with some examples of octagonal and square shapes. The mouldings are generally half-rounds, which overhang deep hollows in the capital. In France, the abacus in early work is generally square, as at Chateau Blois (fig. 3).

Examples in France

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Decorated with simple mouldings and ornaments, particularly during the 12th century, in Isle-de-France, Normandy, Champagne, and Burgundy region (fig. 2, below). Second example, from the chorus of the Vézelay Abbey (fig. 3, below). Circular abacus used at windows in the side chapels of Notre Dame de Paris (fig. 5). Towards the end of the 13 century the abacus decreases in importance - they become low, thin, not very projecting during the 14th century, and disappear almost entirely during the 15th century (fig. 7). Abaque.eglise.Vezelay.png Abaque.circulaire.XIIIe.siecle.png Abaque.XVe.siecle.png

External links

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• Abacus, Smith's Dictionary of Greek and Roman Antiquities

Architectural elements

Abacus (arkitektur) | Abakus (Architektur) | Ábaco (Arquitectura) | Abaque (architecture) | Abaco (architettura) | Abaks | Abacus (architectuur) | Abakus (architektura)