491 in Roman numerals

How CDXCI is built

Roman numerals are written one decimal place at a time, largest first.

1. CDhundreds · 4C before D means 500 − 100 = 400. This saves writing CCCC.
2. XCtens · 9X before C means 100 − 10 = 90. This saves writing LXXXX.
3. Iones · 1I is 1.

400 + 90 + 1 = 491

The pairs CD and XC use subtraction: a smaller letter before a larger one is taken away from it. Only six pairs work this way: IV, IX, XL, XC, CD and CM. More on subtraction

Mistakes to avoid

• Not XDI. It looks like a shortcut, but X may only come before L or C, not D. The correct form is CDXCI.
• Not CCCCLXXXXI (10 letters). That adds everything up without subtraction. The Romans sometimes wrote numbers this way, and clocks still show 4 as IIII, but today CDXCI (5 letters) is the standard.