ABACUS, in architecture, signifies the superior part or member of the capital of a column, and serves as a kind of crowning to both. Vitruvius tells us the abacus was originally intended to represent a square tile laid over an urn, or rather over a basket.—An Athenian old woman happening to place a basket, thus covered, over the root of an acanthus; that plant shooting up the following spring, encompassed the basket all round, till meeting with the tile, it curled back in a kind of scroll. An ingenious sculptor passing by, took the hint, and immediately executed a capital on this plan; representing the brick by the abacus, the leaves by the volutes, and the basket by the vase, or body of the capital. Such was the rise of the first regular order.—The form of the abacus is not the same in all orders: in the Tuscan, Doric, and Ionic, it is generally square; but in the Corinthian and Composite, its four sides are arched inwards, and embellished in the middle with some ornament, as a rose or other flower. Scamozzi uses abacus for a concave moulding on the capital of the Tuscan pedestal; and Palladio calls the plinth above the echinus, or boulder, in the Tuscan and Doric orders, by the same name *.