Convert Arabic numbers to Roman numerals and back.
Seven symbols: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1000. A number is built by placing them from largest to smallest and adding: MCLXVI is 1000 + 100 + 50 + 10 + 5 + 1 = 1166. The system has no zero, which is among the reasons it was abandoned.
When a smaller symbol precedes a larger one it is subtracted rather than added: IV is 4, IX is 9, XL is 40, CM is 900. Only I may be subtracted from V and X, X from L and C, and C from D and M — which is why 99 is XCIX rather than IC. The rule avoids writing four identical symbols in a row.
With the standard seven symbols the largest ordinary number is 3999, written MMMCMXCIX, because M may not repeat more than three times. For larger values the Romans used a bar above a symbol to mean multiplication by a thousand, but that notation is effectively unused today.
It is a clockmaking convention rather than a mathematical error. IIII balances the dial visually against VIII opposite it, and lets every numeral be cast from the same set of strokes. In arithmetic the correct form remains IV, and that is what this calculator uses.