FM 6-08 Chapter 6 · Numbers and units
Roman numerals
Roman numerals to numbers and back, 1 to 3999, with the decomposition and strict checks.
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Usable as a step in ChainLetters: I 1, V 5, X 10, L 50, C 100, D 500, M 1000. Each decimal digit is written on its own, largest first. Only six subtractive pairs exist: IV, IX, XL, XC, CD, CM. I, X and C repeat at most three times, V, L and D never repeat and are never subtracted. Clock faces often show IIII for 4 by tradition; this tool reads only the standard form and rejects IIII.