21 in Roman Numerals
Numbers and numerals are detected as you type. Enter copies, Alt+S swaps.
21 in Roman numerals is XXI. Split 21 by place value: 20 is XX and 1 is I, and XX + I joins into XXI. All its letters are added: 10 + 10 + 1 = 21. In lowercase it is xxi, and the number is twenty-one.
How XXI is built
tens20 = 2 × 10 → XX
20 is two Xs in a row: XX.
ones1 = 1 → I
1 is a single I.
What sets XXI apart
- Order matters
- X comes before I here, so I is added. Swap the two and XIX means 19: the same letters, read as a subtraction.
- Two Xs
- XXI doubles X. A third X is still allowed, and that is XXXI, 31; after it the tens switch to subtraction.
- XX plus one I
- Coming from 20, XX, you only add another I.
Where you see XXI
Sport
Super Bowl XXI was played on January 25, 1987, in Pasadena, where the New York Giants beat the Denver Broncos 39–20. Wikipedia The Games of the XXI Olympiad were the 1976 Summer Olympics in Montreal. Wikipedia The XXI Olympic Winter Games were held in Vancouver in 2010. Wikipedia
Kings, queens and popes
Pope John XXI (1276–1277) is the only Portuguese pope. Wikipedia
Systems numbered with XXI
In most traditional tarot decks, The World is Major Arcana card XXI. Wikipedia
Roman calendar and Latin
In Latin, the number 21 is vīgintī ūnus or ūnus et vīgintī. Wiktionary
Numbers near 21
