What is (a) The uppermost member or division of the capital of a column, immediately under the architrave. See Column. (b) A tablet, panel, or compartment in ornamented or mosaic work called?

abacus — (Arch.) (a) The uppermost member or division of the capital of a column, immediately under the architrave. See Column. (b) A tablet, panel, or compartment in ornamented or mosaic work. (Webster's 1913.)

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