How Roman numerals work
They use combinations of seven letters (I=1, V=5, X=10, L=50, C=100, D=500, M=1000). When a smaller-value letter comes before a larger one, it's subtracted (e.g. IV = 4); when it comes after, it's added (e.g. VI = 6). Only numbers from 1 to 3999 can be represented with standard notation.
Where they're used today
Roman numerals still show up in book chapters, monarch and pope names (e.g. Elizabeth II), centuries (e.g. 21st century, XXI), and movie end credits.
Frequently asked questions
Why is the limit 3999?
Because standard Roman notation has no symbol for 5000 without special conventions (like a bar over the letter) — 3999 (MMMCMXCIX) is the largest number representable with the seven basic symbols.
Is there a zero in Roman numerals?
No — the classic Roman system had no symbol for zero.