Calendar year
 
Year 431 BC was a year of the pre-Julian Roman calendar. At the time, to Romans it was known as the Year of the Consulship of Cincinnatus and Mento (or, less frequently, year 323 "Ab urbe condita"). The denomination 431 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
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