Sentence:  The <WSD>abacus</WSD> is moulded in three sections and has four main concave faces corresponding with the tapering volutes below and truncated by a short sqaure face on the diagonal. 
Meaning of the word marked with <WSD> tags:  The uppermost portion of the capital of a column, immediately under the architrave. (First attested in the mid 16th century.)<ref name=SOED/>