Common Roman Numeral Mistakes
A practical guide to noncanonical and invalid forms—verified against this site’s conversion engine.
Quick comparison table
| Incorrect | Canonical | Value | Why |
|---|---|---|---|
| IIII | IV | 4 | Four is normally written with the subtractive pair IV. Details |
| VIIII | IX | 9 | Nine uses IX rather than seven units after V. Details |
| XXXX | XL | 40 | Forty uses XL (50 − 10), not four tens. |
| LXXXX | XC | 90 | Ninety uses XC (100 − 10), not L followed by four tens. |
| CCCC | CD | 400 | Four hundred uses CD (500 − 100). |
| DCCCC | CM | 900 | Nine hundred uses CM (1000 − 100). |
| IL | XLIX | 49 | I cannot subtract directly from L; 49 is XL + IX. Details |
| IC | XCIX | 99 | I cannot subtract directly from C; 99 is XC + IX. Details |
| XD | CDXC | 490 | X cannot subtract directly from D; 490 is CD + XC. |
| XM | CMX | 910 | X cannot subtract directly from M. |
| VV | X | 10 | V cannot be repeated; ten is X. |
| LL | C | 100 | L cannot be repeated; one hundred is C. |
| DD | M | 1000 | D cannot be repeated; one thousand is M. |
Why IIII is usually IV
Modern canonical teaching writes 4 as IV. Additive IIII is widely recognized and appears on many clock faces, but converters on this site standardize on IV. Full discussion: IIII vs IV.
Why IL is not 49
I may only subtract from V or X (IV, IX). L is not a valid subtractive partner for I. Forty-nine is XLIX. See IL vs XLIX.
Why IC is not 99
I cannot subtract from C. Ninety-nine is XCIX. See IC vs XCIX.
Which symbols can repeat
I, X, C, and M may appear up to three times in a row in canonical form. V, L, and D do not repeat.
Valid subtractive pairs
Only six: IV, IX, XL, XC, CD, CM. Guide: subtractive pairs.
Clock-face IIII
Many dials use IIII for hour 4. That is a clock convention; it is not the canonical modern form used by this site’s converter. See clock numerals.