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784 (DCCLXXXIV) was a leap year starting on Thursday of the Julian calendar, the 784th year of the Common Era (CE) and Anno Domini (AD) designations, the 784th year of the 1st millennium, the 84th year of the 8th century, and the 5th year of the 780s decade. As of the start of 784, the Gregorian calendar was 4 days ahead of the Julian calendar, which was the dominant calendar of the time.

• The Japanese capital moved away from Nara. End of the Nara period.
• The Chinese engineer and prince Li Gao invents a successful model of a paddle-wheel ship.
• August 31—Paul IV abdicates as Patriarch of Constantinople
• December 25—Tarasius elected Patriarch of Constantinople

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