The Roman numeral for the number 13, using the symbols X (10), I (1), I (1), and I (1) arranged as 13.
From the Roman numeral system used by ancient Romans. XIII is literally X (10) + III (3), combining the symbols for 10 and 3 to make 13.
Roman numerals seem mysterious until you realize they're basically tally marks made fancy—Romans were literally just adding I's and X's together, which is why IV (4) looks backward compared to VI (6).
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