What is the longest roman numeral under 1000 why?

Answer 1

The longest Roman numeral under 1000 is CMXCIX, which represents the number 999. This is the longest because it combines the largest Roman numeral symbols in descending order from left to right: CM for 900, XC for 90, and IX for 9. Roman numerals are additive, so the longest numeral is typically one that requires the most symbols to represent a number.

Answer 2

Under today's rules now governing the Roman numeral system it is DCCCLXXXVIII = 888 and we now considered 999 to be CMXCIX but the ancient Romans would have worked out the equivalent of 999 as DCCCCLXXXXVIIII which can be abridged to IM meaning -1+1000 = 999