991 in Roman Numerals
991
C (subtracted)MX (subtracted)CI
nine hundred ninety-one
How CMXCI is formed
| Place | Value | Symbols |
|---|---|---|
| Hundreds | 900 | C (subtracted)M |
| Tens | 90 | X (subtracted)C |
| Units | 1 | I |
| Total | 991 | CMXCI |
CMXCI is written one place at a time, from the largest to the smallest: CM (900) + XC (90) + I (1) = 991.
CM and XC are subtractive pairs: in each one the smaller symbol comes first and is taken away.
Common mistakes
Wrong: DCCCCXCI
Why isn't 991 written as DCCCCXCI?
A symbol can't be repeated four times in a row. 900 is CM (1000 − 100), not DCCCC, so 991 is CMXCI.
Wrong: CMLXXXXI
Why isn't 991 written as CMLXXXXI?
A symbol can't be repeated four times in a row. 90 is XC (100 − 10), not LXXXX, so 991 is CMXCI.
Wrong: DCCCCLXXXXI
Why isn't 991 written as DCCCCLXXXXI?
No symbol can be repeated four times in a row, so DCCCC and LXXXX are not used: 900 = CM, 90 = XC. Together they make CMXCI.
Numbers near 991
Properties of 991
- Prime factors
- 991
- Divisors
- 1, 991
- Even or odd
- Odd
- Prime number
- Yes
- Letters in CMXCI
- 5
991 is a prime number: it can only be divided by 1 and by itself.
991,000 in Roman numerals
With a bar over it (vinculum), C̅M̅X̅C̅I̅ means 991 × 1,000 = 991,000. How Roman numerals work