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I don't have specific context about what "721" refers to. To provide an accurate 2-sentence overview, I would need more information about whether you're asking about a year, a historical event, a law, a product, a place, or something else entirely.
AI-generated from the Wikipedia summary — may contain errors.
Article
Přemysl and Libuše statue (Vyšehrad)
Year 721 (DCCXXI) was a common year starting on Wednesday of the Julian calendar. The denomination 721 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.