Seven Symbols, Additive and Subtractive

Roman numerals use just seven symbols — I (1), V (5), X (10), L (50), C (100), D (500), M (1,000) — combined by two rules: add when symbols descend (VI = 6), subtract when a smaller one precedes a larger one (IV = 4). The converter applies both rules in both directions, so 1,988 becomes M + CM + LXXX + VIII = MCMLXXXVIII.

Why Subtract?

Subtractive notation (IX instead of VIIII) was the Romans' compression scheme: it shortens numerals and removes ambiguity. The rules are strict — only I, X, and C may precede the next two symbols up (IV, IX, XL, XC, CD, CM) — which is why sequences like IIV or VX are invalid. The converter enforces these exact rules when parsing.

The 3,999 Ceiling

With the standard seven symbols, the largest writable number is 3,999 (MMMCMXCIX). Larger numbers required the vinculum — a bar over a symbol multiplying it by 1,000 — which complicates everyday notation. For dates, years, and classic numbering, 1–3,999 covers virtually every practical use, and that's the converter's accepted range.