399 in Roman numerals

How CCCXCIX is built

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

1. CCChundreds · 3C written 3 times: 3 × 100 = 300.
2. XCtens · 9X before C means 100 − 10 = 90. This saves writing LXXXX.
3. IXones · 9I before X means 10 − 1 = 9. This saves writing VIIII.

300 + 90 + 9 = 399

The pairs XC and IX 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 CCCIC. It looks like a shortcut, but I may only come before V or X, not C. The correct form is CCCXCIX.
• Not CCCLXXXXVIIII (13 letters). That adds everything up without subtraction. The Romans sometimes wrote numbers this way, and clocks still show 4 as IIII, but today CCCXCIX (7 letters) is the standard.