How to Read Roman Numerals, and Where They're Used
Roman numerals are built from seven basic letters: I (1), V (5), X (10), L (50), C (100), D (500), and M (1000). Unlike the decimal system we use today, in this system a number's value depends on the order of the letters and their position relative to one another. If a letter comes before a larger letter, it's subtracted; if it comes after, it's added. For example, IV means 5-1=4 because I (1) comes before V (5); VI means 5+1=6 because I comes after V. This rule is what makes combinations like IX (9), XL (40), and XC (90) readable.
There are a few common places where Roman numerals still show up in everyday life. On clock faces (especially classically designed wall clocks), the hours are often shown in Roman numerals. In movie and TV credits, the production year is often written in Roman numerals (like MMXXIV). Roman numerals are also used for a classic look in book prefaces, volumes, and chapter numbers. They're also standard for numbering royal or monarchic succession (Elizabeth II, Henry VIII) and major sporting events (like the Super Bowl).
A common mistake when writing Roman numerals is repeating the same letter more than three times in a row; under the standard rules a letter can be written at most three times in a row (III=3 is valid, but IV is expected instead of IIII=4). Also, the subtraction rule only applies to certain letter pairs (I can only be subtracted before V and X, X only before L and C, C only before D and M); combinations outside these rules aren't considered standard Roman numerals.
You can find ready-made Roman numeral characters among the symbol categories on this site and copy them with one click; these characters are separate Unicode code points, meaning you can use the ready-made symbol directly instead of combining the letters I, V, X one by one.