In classical architecture the abacus is a flat slab that sits upon the capital of a column, forming the transition between the column and the architrave it supports.
The shape of the abacus and its edge profile varies in the different architectural orders. In the Greek Doric it is simply a square unmolded slab. In Roman Doric, the upper corner bears a crown molding. In the Greek Ionic the slab of the abacus is thinner and its profile in molded as an ovolo. In Roman Ionic and Corinthian examples, its sides are concave and its corners cut at an angle, to suit the outscrolling volutes or acanthus leaves.
In Romanesque architecture the abacus survives as a heavier slab, generally molded and decorated.
In Gothic architecture the molded forms of the abacus vary; it may even be a flat disk or drum. The Gothic abacus is often affected by the shaping of a vault that springs from the column, in which case it might better be called an impost block.
