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Roman numerals use subtractive notation: a smaller symbol before a larger one is subtracted, so MCMXCIV is 1000 + (1000-100) + (100-10) + (5-1) = 1994. Building the other direction works greedily through the value table, which is why 2026 becomes MMXXVI. The system has no zero and conventionally stops at 3999.
Subtractive rule
a smaller symbol before a larger one is subtracted, e.g. IX = 9
Symbol values
I 1, V 5, X 10, L 50, C 100, D 500, M 1000
Because 4000 would need four Ms, which the standard subtractive form forbids. Larger numbers used an overbar meaning times a thousand.
It appears on clock faces for visual balance, but standard notation requires IV. Both were used historically.