Common Roman numeral mistakes
Most wrong Roman numerals break one of two rules: which letter may be subtracted from which, and how many times a letter may repeat. Here are five common mistakes, and IIII, which breaks the second rule but is an older convention rather than a mistake.
Five mistakes and a convention
| Written | Standard form | Value | Why |
|---|---|---|---|
| IC | XCIX | 99 | In the standard form a letter is subtracted only from the next two values above it. I can come before V or X, and nothing else. |
| IL | XLIX | 49 | Same rule. Forty-nine is forty then nine, written as two separate subtractions. |
| VX | V | 5 | In the standard form V, L and D are never subtracted. They already represent a five, and taking one five from a ten has no meaning in the system. |
| IIII | IV | 4 | Not standard, but not a mistake either: an older convention, still usual on clock faces. The standard form keeps to three in a row, which is what the subtractive pairs exist for. |
| MDCCCCX | MCMX | 1910 | Four Cs in a row. Nine hundred is CM, not CCCC. |
| XXXX | XL | 40 | Forty is XL. The same three-in-a-row limit. |
The subtraction rule
In the standard form, a letter may only be subtracted from the next two values above it, and only I, X and C are subtracted. That gives six pairs: IV, IX, XL, XC, CD and CM. Everything else is ruled out. Roman inscriptions are less tidy: IIX for eight and XIIX for eighteen both occur in them, and are simply not the form a reader now expects.
IC looks reasonable if you think of subtraction as arithmetic: a hundred take away one is ninety-nine. But the numeral is a spelling, and the spelling works column by column. Ninety-nine is ninety and nine, XC then IX, so it is XCIX.
IIII is not really a mistake
IIII was in ordinary use for centuries and is still on the face of almost every traditional clock, where IV would sit awkwardly opposite VIII and where the repeated I balances the dial. Roman inscriptions use both forms, sometimes in the same text.
So IIII is not wrong in the way IC is wrong. It is a different, older convention, and the only reason to avoid it is that the modern standard form is what a reader now expects. The converter reads IIII and answers with IV.
Things that are not numerals at all
- A lowercase numeral. Perfectly correct: lowercase roman numerals number the front matter of books and the items in an outline. They mean the same as the capitals.
- The letter O for zero. There is no zero. The system has no symbol for it.
- A numeral with a J on the end. An old scribal habit for the final I in a number, as in vij for seven, used in medieval manuscripts and prescriptions. Not modern usage.
- Anything above MMMCMXCIX. Larger numbers need an overline, which is not part of the standard form.
How to check one quickly
Read it left to right. The values should generally fall or stay level; where one rises, the letter before it must be I, X or C, and the rise must be to the next two values up. If a letter appears four times in a row, or a V, L or D sits before something larger, it is not standard.
Or paste it into the converter, which reads nonstandard spellings and tells you what the modern form would be.