Its chief function is to provide a large supporting surface, tending to be wider than the capital, as an abutment 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 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 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.
The same shape is adopted in the Roman and Renaissance Corinthian and Composite capitals, in some cases with the carved ovolo moulding, fillet, and cavetto.
In Gothic 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.
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 of Vézelay Abbey (fig.