ABACUS, in architecture, the upper member of a column capital whose function is to provide a supporting surface for the structure above. In Egyptian capitals it is smaller than the parts below and in the late work it has great height. In the Greek Doric order it is a plain square slab, but in the Roman Doric it is usually crowned by a moulding. In the Greek Ionic order it is at first rec tangular, to cap the projection of the volutes, but later becomes square, except at corner capitals where it is curved out over the angle volutes. In both Greek and Roman Corinthian orders and in the composite and four-sided or Scamozzi Ionic order it is moulded with concave sides and usually the corners are cut off slightly. In Romanesque architecture the abacus usually is square, with sides splayed or moulded, and in the Gothic, outside of Eng land, it is square or octagonal and richly moulded. In English Gothic, however, circular forms are frequent, especially in 13th century work. The diminutive, abaciscus, is applied to the squares of a tessellated pavement.