In architecture, an abacus 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, 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.
1898 illustration of abacuses of many capitals in various styles
Abacus of the Rampurva capital from the time of Emperor Ashoka, 3rd century BCE, India
In architecture the capital or chapiter forms the topmost member of a column. It mediates between the column and the load thrusting down upon it, broadening the area of the column's supporting surface. The capital, projecting on each side as it rises to support the abacus, joins the usually square abacus and the usually circular shaft of the column. The capital may be convex, as in the Doric order; concave, as in the inverted bell of the Corinthian order; or scrolling out, as in the Ionic order. These form the three principal types on which all capitals in the classical tradition are based. The Composite order established in the 16th century on a hint from the Arch of Titus, adds Ionic volutes to Corinthian acanthus leaves.
Illustration of papyriform capitals, in The Grammar of Ornament, 1856
Nine types of capitals, from The Grammar of Ornament
Columns with Hathoric capitals, at the Temple of Isis from island Philae
Egyptian composite columns from Philae