abacus

1898 illustration of abacuses of many capitals in various styles In architecture, an abacus (from the Ancient Greek ἄβαξ (ábax), 'slab'; or French abaque, tailloir; pl.: abacuses or abaci) is a flat slab forming the uppermost member or division of the capital of a column, above the bell. Its chief function is to provide a large supporting surface, tending to be wider than the capital, as an abutment to receive the weight of the arch or the architrave above. The diminutive of abacus, abaculus, is used to describe small mosaic tiles, also called abaciscus or tessera, used to create ornamental floors with detailed patterns of chequers or squares in a tessellated pavement.

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