21 in Roman numerals at a glance
- Roman numeral
- XXI
- Lowercase
- xxi
- Letters
- 3
- Parts
- X · X · I
- Subtractive pairs
- None
- In words
- twenty-one
- Latin name
- viginti unus
How XXI is built
Roman numerals are written one decimal place at a time, from the largest place to the smallest. Converting each digit of 21 separately and joining the pieces gives XXI:
| Place | Digit | Value | Numeral |
|---|---|---|---|
| Tens | 2 | 20 | XX |
| Ones | 1 | 1 | I |
Reading it back, left to right:
- X = 10 → running total 10
- X = 10 → running total 20
- I = 1 → running total 21
The rules XXI follows
- Largest to smallest. XXI uses the symbols X and I (3 characters in all), and the values never increase from left to right except inside a subtractive pair.
- No subtraction needed. None of the digits of 21 is a 4 or a 9, so XXI is purely additive: just add the symbol values.
- Repetition. XX repeats X two times. I, X, C and M may repeat up to three times in a row; V, L and D never repeat.
Where you will see XXI
- The U.S. Constitution. Amendments are often cited in Roman numerals: Amendment XXI repealed Prohibition (ratified in 1933).
- The modern Summer Games. They are officially numbered by Olympiad; the Games of the XXI Olympiad belong to 1976: Montreal.
- Championship games. Pro football numbers its championship game with Roman numerals, starting from game I in January 1967. Game XXI was played in early 1987.
- Volumes and sequels. Book volumes, conference editions, film sequels and anniversary events often carry a Roman numeral; edition XXI would be the 21st.
Numbers near 21 in Roman numerals
Frequently asked questions
How do you write 21 in Roman numerals?
21 is written XXI. Break it into X (10), X (10), I (1) and add the parts: 10 + 10 + 1 = 21.
What is 21 in Latin?
The Latin cardinal number 21 is “viginti unus.” The Romans wrote it as XXI.
What is XXI plus one?
XXI + I = XXII (22). Adding one can change the whole ending of a numeral, which is why it helps to convert to digits, add, and convert back.