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Roman Numerals Converter

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ReferenceIVXLCDM
=1510501005001000

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Numbers to Roman numerals and back, 1 to 3999 — strict about the canonical subtractive forms, so IIII and IC are refused with an explanation instead of being silently 'understood'.

MCMXC, read like a Roman

Reading is grouping: M CM XC is 1000 + 900 + 90 = 1990. The subtractive pairs are the whole difficulty, and there are only six of them — IV, IX, XL, XC, CD, CM. Everything else is repetition capped at three.

Writing goes greedy from the top: subtract the largest value the table allows, append its glyph, repeat. That algorithm cannot produce a non-canonical form, which is why the decoder simply re-encodes its answer and compares — a two-line proof of validity.

Frequently asked questions

Why is IIII rejected when clocks use it?

Clock faces are the one traditional exception; everywhere else — chapters, monarchs, film credits, dates on buildings — the canonical form is IV. A converter that accepts IIII will also accept VV and IC, and at that point it is teaching mistakes. Strictness here is the feature.

Why does it stop at 3999?

Because 4000 would need MMMM or the overline notation (a bar multiplying by 1000), and the overline does not survive plain text. Every practical use — years, chapters, outlines — lives comfortably below the ceiling.

What are the subtractive rules exactly?

Only I, X and C may be subtracted, only from the next two steps up: IV and IX; XL and XC; CD and CM. Never V, L or D on the left, never a jump like IC (99 is XCIX). The validator here enforces exactly that list — and double-checks by converting back.

How do I type a year like 2026 for a tattoo or a plaque?

MMXXVI — and double-check it here in both directions before anything permanent. The classic mistakes are ordering (XXMVI) and the seductive-but-wrong subtractions; the strict decoder refuses exactly those.

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