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Inarchitecture, anabacus (from theAncient Greekἄβαξ (ábax),'slab'; orFrenchabaque,tailloir;pl.:abacuses orabaci)[1] is a flat slab forming the uppermost member or division of thecapital of acolumn, above thebell. Its chief function is to provide a large supporting surface, tending to be wider than the capital, as anabutment to receive the weight of thearch or thearchitrave above. The diminutive of abacus,abaculus, is used to describe smallmosaic tiles, also calledabaciscus ortessera, used to create ornamental floors with detailed patterns of chequers or squares in atessellated pavement.
Inclassical architecture, the shape of the abacus and its edge profile varies in the differentclassical orders. In the GreekDoric order, the abacus is a plain square slab withoutmouldings, supported on anechinus.[2] In the Roman and Renaissance Doric orders, it is crowned by a moulding (known as "crown moulding"). In theTuscan and Roman Doric capital, it may rest on aboltel.
In the archaic GreekIonic order, owing to the greater width of the capital, the abacus is rectangular in plan, and consists of a carvedovolo moulding. In later examples, the slab is thinner and the abacus remains square, except where there are angledvolutes, where the slab is slightly curved. In the Roman and Renaissance Ionic capital, the abacus is square with a fillet on the top of anogee moulding with curved edges over angled volutes.[2]
In an angular capital of the GreekCorinthian order, the abacus is moulded, its sides are concave, and its angles canted[3] (except in one or two exceptional Greek capitals, where it is brought to a sharp angle); the volutes of adjacent faces meet and project diagonally under each corner of the abacus. The same shape is adopted in the Roman and Renaissance Corinthian andComposite capitals, in some cases with the carved ovolo moulding,fillet, andcavetto.[2][4]
InRomanesque 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; but in England, inEarly English work, a circular deeply moulded abacus was introduced, which in the 14th and 15th centuries was transformed into an octagonal one.
InGothic architecture, the moulded forms of the abacus vary in shape, such as square, circular, or even octagonal,[5] it may even be a flat disk or drum.[2] The form of the Gothic abacus is often affected by the shape of a vault that springs from the column, in which case it is called animpost block.
Inśilpaśāstra, the ancient Indian science of sculpture, the abacus is commonly termed asphalaka (orphalakā).[6] It consists of a flat plate and forms part of the standard pillar (stambha). Thephalaka should be constructed below thepotikā ("bracket"). It is commonly found together with the dish-like maṇḍi as a single unit. The term is found in encyclopedic books such as theMānasāra,Kāmikgāgama and theSuprabhedāgama.
EarlySaxon abaci are frequently simplychamfered, but sometimes grooved as in the crypt atRepton (fig. 1) and in the arcade of the refectory atWestminster Abbey. The abacus in Norman work is square where the columns are small; but on larger piers it is sometimes octagonal, as atWaltham Abbey. The square of the abacus is often sculptured withornaments, as at theWhite Tower and atAlton, 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 atChateau de Blois (fig. 3).
The first abacus pictured below (fig. 5) is decorated with simple mouldings and ornaments, common during the 12th century, in Île-de-France, Normandy, Champagne, and Burgundy regions, and from the choir ofVézelay Abbey (fig. 6). Figure 7 shows a circular abacus used at windows in the side chapels ofNotre Dame de Paris. Towards the end of the 13th century, this element decreases in importance—they became short with a narrow profile during the 14th century, and disappeared almost entirely during the 15th century (fig. 8).
