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I don't have sufficient context to write an accurate overview of what "1644" refers to. The context provided only indicates it's a year, but without additional information about which specific historical event, person, work, or topic you're asking about, I cannot write a factually accurate overview. Could you clarify what aspect of 1644 you'd like me to focus on?
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May 27: The Battle of Shanhai Pass helps bring the Manchu dynasty to power in China for two and a half centuries. July 1: The Battle of Colberger Heide is fought between Sweden and Denmark-Norway off of the German coast. July 2: The Battle of Marston Moor gives Parliament control of northern England from King Charles.
1644 (MDCXLIV) was a leap year starting on Friday of the Gregorian calendar and a leap year starting on Monday of the Julian calendar, the 1644th year of the Common Era (CE) and Anno Domini (AD) designations, the 644th year of the 2nd millennium, the 44th year of the 17th century, and the 5th year of the 1640s decade. As of the start of 1644, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923. It is one of eight years (CE) to contain each Roman numeral once (1000(M)+500(D)+100(C)+(-10(X)+50(L))+(-1(I)+5(V)) = 1644).