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I don't have enough context to write an accurate overview. The term "421" could refer to many things (a law, regulation, historical event, mathematical concept, etc.), and you've only provided "year" as context, which isn't sufficient to determine what "421" means or why it matters. Could you provide more information about what "421" refers to?
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Solidus of Emperor Constantius III
Year 421 (CDXXI) was a common year starting on Saturday of the Julian calendar. At the time, it was known as the Year of the Consulship of Agricola and Eustathius (or, less frequently, year 1174 Ab urbe condita). The denomination 421 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.