Roman Numeral Subtractive Pairs
The six valid subtractive pairs: IV, IX, XL, XC, CD, and CM.
| Pair | Value | Meaning |
|---|---|---|
| IV | 4 | V − I |
| IX | 9 | X − I |
| XL | 40 | L − X |
| XC | 90 | C − X |
| CD | 400 | D − C |
| CM | 900 | M − C |
Pairs that are not allowed
Forms such as IL, IC, XD, and XM are invalid because they use subtractive partners outside the six pairs above. Check any string with the checker.
Full table: common mistakes. Rules overview: rules.