Roman numerals

How Roman numerals work

Roman numerals are the number system of ancient Rome, which writes numbers with seven letters: I (1), V (5), X (10), L (50), C (100), D (500) and M (1000). Values are added from left to right, largest first: MDCLXVI = 1000 + 500 + 100 + 50 + 10 + 5 + 1 = 1666.

To avoid four identical letters in a row, a smaller letter placed before a larger one is subtracted. Only six such pairs are used: IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900). So 1994 is M + CM + XC + IV = MCMXCIV. With these rules the largest number is 3,999, MMMCMXCIX; there is no zero.

Where Roman numerals is used

• Years on buildings, film credits and copyright notices (MMXXIV).
• Clock and watch faces, where 4 is often written IIII.
• Names of monarchs and popes (Elizabeth II, Benedict XVI) and events such as the Super Bowl (Super Bowl LVIII).
• Book chapters, outlines, prefaces (page numbers i, ii, iii) and music theory (chord numerals).

History

Roman numerals grew out of Etruscan tally marks and were used across the Roman Empire and medieval Europe. The subtractive forms such as IV became standard only gradually; Romans themselves often wrote IIII. They were replaced for arithmetic by Hindu-Arabic numerals from the 13th century on, but remain in use for decoration and numbering.