Which is correct for 1999 in Roman numerals MCMXCIX or IMM and why?

Answer 1

Either way is correct - however IMM should be written MIM. Working from the right-hand side - if the next letter to the left is lower - you subtract it.

Improved Answer:-

In accordance to the rules governing the Roman numeral system 1999 in Roman numerals is generally reckoned to be MCMXCIX. But these rules were made during the Middle Ages and centuries after when the Romans were no longer around for reference purposes.

Notwithstanding the above there's strong evidence to support the fact that the Romans themselves would have actually calculated 1999 on an abacus counting device as MDCCCCLXXXXVIIII and then by placing I to both sides of these numerals probably simplified them to IMM (-1+2000=1999)

It's interesting to note that when MCMXCIX is set out in the format of M+CM+XC+IX they total up to IMM because the Romans could have possibly added them up in the following manner:-

M+CM = CMM

CMM+XC = XMM

XMM+IX = IMM

Hindu-Arabic conversion:-

1000+900 = 1900

1900+90 = 1990

1990+9 = 1999

Roman numerals: M=1000, D=500, C=100, L=50, X=10, V=5 and I=1