84 in Roman Numerals

Roman numerals are built from seven letters — I (1), V (5), X (10), L (50), C (100), D (500) and M (1000) — written from largest to smallest and added up, so MDCLXVI is 1000 + 500 + 100 + 50 + 10 + 5 + 1 = 1666. To avoid four of the same letter in a row, the modern subtractive rule places one smaller letter before a larger one to mean "subtract": IV is 4, IX is 9, XL is 40, XC is 90, CD is 400 and CM is 900. Only those six pairs are allowed, and only powers of ten (I, X, C) are ever subtracted — which is why 99 is XCIX, not IC. The older additive style wrote every letter out (IIII for 4, VIIII for 9); you still see IIII for four on many clock faces, kept for visual balance opposite the VIII. There is no classical symbol for zero — Roman counting had no need for it — though medieval scholars used the word "nulla" (N). Standard numerals reach 3999 (MMMCMXCIX); beyond that the vinculum, a bar drawn over a numeral, multiplies it by 1000, so an overlined V is 5000 and an overlined M is 1,000,000, extending the system to 3,999,999. This converter reads all of these forms — standard, additive or overlined, in any case — and always shows the clean modern spelling alongside a full symbol-by-symbol breakdown.