Editing Abacus (architecture) (section)

== Definition ==
In [[classical architecture]], the shape of the abacus and its edge profile varies in the different [[classical order]]s. In the Greek [[Doric order]], the abacus is a plain square slab without [[molding (decorative)|moulding]]s, supported on an [[Echinus (molding)|echinus]].<ref name=fletcher1>{{harvnb|Cruickshank|1996|p=1713}}</ref> In the Roman and Renaissance Doric orders, it is crowned by a moulding (known as "[[crown molding|crown moulding]]"). In the [[Tuscan order|Tuscan]] and Roman Doric capital, it may rest on a [[Bowtell|boltel]].

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 [[volute]]s, 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.<ref name=fletcher1/>

[[Image:Abacus architecture.gif|frame|center|Individual sections of the column and as constructed by ancient [[Egyptians]], [[ancient Greece|Greeks]] and [[Ancient Rome|Romans]]]]
In an angular capital of the Greek [[Corinthian order]], the abacus is moulded, its sides are concave, and its angles canted<ref>{{harvnb|Avery|1962|p=1}}</ref> (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 and [[Composite order|Composite]] capitals, in some cases with the carved ovolo moulding, [[Annulet (architecture)|fillet]], and [[wikt:cavetto|cavetto]].<ref name=fletcher1/><ref>{{harvnb|Kay|1955|p=1}}</ref>

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; but in England, in [[Early English Period|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 in shape, such as square, circular, or even octagonal,<ref>{{harvnb|Lagassé|2000|p=1}}</ref> it may even be a flat disk or drum.<ref name=fletcher1/> 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 an [[impost (architecture)|impost block]].