Roman numerals survive on clock faces, movie credits, Super Bowls and monarchs — a 2,000-year-old system still in light production use. This converter runs both directions with strict validation, so MCMXCIV decodes to 1994 and "IIII" gets flagged rather than guessed at.
How the system composes
Symbols add left to right, largest first: MMXXVI = 2000 + 20 + 6 = 2026. The twist is subtraction: exactly six pairs (IV 4, IX 9, XL 40, XC 90, CD 400, CM 900) place a smaller symbol first to avoid four-in-a-row. That's why 1994 is MCMXCIV — M + CM + XC + IV — and the breakdown line shows every piece.
Strict rules, honestly enforced
Only those six subtractive pairs are valid: 45 is XLV, never VL; 99 is XCIX, never IC. No symbol repeats four times, and V, L, D never repeat at all. The validator applies the full grammar, which is why sloppy inscriptions from the actual Romans (they were inconsistent!) sometimes fail it — the standard form is a later formalization.
Clock faces and other exceptions
Traditional clocks show IIII instead of IV — visual balance with VIII across the dial, a convention older than the strict rules. And there's no zero: the system predates the concept as a numeral, one reason positional systems (with zero as placeholder) eventually won arithmetic. See how positional notation works in any base in the number base converter.
Why 3,999 is the ceiling
MMMM is disallowed and no standard symbol exceeds M, so standard notation caps at MMMCMXCIX. Romans wrote bigger numbers with a vinculum — an overbar multiplying by 1,000 — but modern usage (years, sequels, kings) never needs it.
Strict modern notation; both conversions validated.