Sentence:  I've heard merchants still use an <WSD>abacus</WSD> for adding things up in China. 
Meaning of the word marked with <WSD> tags:  A calculating table or frame; an instrument for performing arithmetical calculations by balls sliding on wires, or counters in grooves, the lowest line representing units, the second line, tens, etc. (First attested in the late 17th century.)<ref name=SOED>(page=2)</ref>