An illustration of a Byzantine capital from the the Church of St. Vitale, Ravenna. In several traditions of architecture including Classical architecture, the capital (from the Latin caput, ‘head’) forms the crowning member of a column or a pilaster. The capital projects on each side as it rises, in order to support the abacus and unite the form of the latter (normally square) with the circular shaft of the column.
Encyclopaedia Britannica (11th ed., vol. 5) (New York, NY: The Encyclopaedia Britannica Company, 1910)