Harry Thurston Peck, Harpers Dictionary of Classical Antiquities (1898)

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Cyăthus

（κύαθος). A Greek and Roman liquid measure, containing one twelfth of the sextarius or .0825 of a pint English. It was, in later times at least, the measure of the common drinking-glass

Cyathi. ( Museo Borbonico. ）

among the Romans, who borrowed it from the Greeks. The form of the cyathus used at banquets was that of a small ladle, by means of which the wine was conveyed into the drinking-cups from the large vessel (κρατήρ) in which it was mixed.

The cyathus was the uncia, considered with reference to the sextarius as the unit; hence we have sextans used for a vessel containing the sixth of the sextarius, or two cyathi, quadrans for one containing three cyathi, triens for four cyathi, quincunx for five cyathi, and so on.