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I don't have enough context to write an accurate overview of "644." The word "644" could refer to a year, a numerical code, a file permission system, or something else entirely. Without additional information about what specifically "644" refers to, I cannot provide a reliable 2-sentence explanation.
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Article
King Oswine of Deira (644–651)
Year 644 (DCXLIV) was a leap year starting on Thursday of the Julian calendar. The denomination 644 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.