Roman Numeral Converter

Seven letters carry values (I=1, V=5, X=10, L=50, C=100, D=500, M=1000), written largest to smallest and added, except that a smaller value before a larger one subtracts: IV is 4, IX is 9, XL is 40, CM is 900.

Subtractive notation is the standard modern convention. IIII survives on clock faces as a deliberate tradition, and the converter flags it as non-standard rather than wrong, showing the canonical IV.

Because 4000 would need MMMM or an overlined IV, and the overline does not exist in plain text. 3999, MMMCMXCIX, is the honest ceiling of the plain-text system.

MMXXVI. And 1994, the perennial example, is MCMXCIV, which exercises every subtractive pair.