abacus [key], in architecture, flat slab forming the top member of a capital. In classical orders it varies from a square form having unmolded sides in the Greek Doric, to thinner proportions and ovolo molding in the Greek Ionic, and to sides incurving and corners cut in Roman Ionic and Corinthian examples. In Romanesque work the abacus is heavier in proportion, projects less, and is generally molded and decorated. In Gothic work the form varies, appearing in square, circular, and octagonal forms with molded members.
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