Write 999 in Roman numerals.

CMXCIX

To write 999 in Roman numerals, we need to break it down into smaller values using subtraction, as Roman numerals follow specific rules.

1. 999 = 900 + 90 + 9
   We split the number into three parts to simplify it.
2. 900 is written as CM:
   • C = 100, M = 1000.
   • When a smaller value (C) appears before a larger value (M), it is subtracted.
   • So, 900 = 1000 - 100 = CM.
3. 90 is written as XC:
   • X = 10, C = 100.
   • Here, the smaller value (X) comes before the larger value (C), so it is subtracted.
   • So, 90 = 100 - 10 = XC.
4. 9 is written as IX:
   • I = 1, X = 10.
   • Again, the smaller value (I) is placed before the larger value (X), meaning it is subtracted.
   • So, 9 = 10 - 1 = IX.

Now, combine all three parts:
999 = 900 + 90 + 9 = CM + XC + IX = CMXCIX