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I don't have enough context to provide an accurate overview of "842." The only information provided is that it relates to a year, but without additional context about what specifically "842" refers to (an event, law, historical period, etc.), I cannot write an accurate and factual overview.
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Empress Theodora with her son Michael III
Year 842 (DCCCXLII) was a common year starting on Sunday of the Julian calendar, the 842nd year of the Common Era (CE) and Anno Domini (AD) designations, the 842nd year of the 1st millennium, the 42nd year of the 9th century, and the 3rd year of the 840s decade.