The subtractive rule
A smaller symbol before a larger one is subtracted: IV is 4, IX is 9, XL is 40, CM is 900. Only I, X and C are used this way, and only before the next two values up — so 99 is XCIX and never IC, which is a common mistake in tattoos and clock faces alike.
No symbol repeats more than three times, which is why 40 is XL rather than XXXX.
Limits, and the clock face exception
There is no zero and no way to write a negative or a fraction, and the standard symbols stop at 3,999 — above that you need overlines meaning "multiply by a thousand", which most software cannot render.
Clock faces very often show IIII rather than IV for four. It has been the convention in clockmaking for centuries, for reasons of visual balance, and it is not an error.
Frequently asked questions
Why is 99 XCIX and not IC?
Subtraction only works with the next two values up. 99 is 90 (XC) plus 9 (IX). IC is a common mistake and is not valid.
What is the largest number?
3,999 with standard symbols. Larger values need overlines meaning "times a thousand", which almost nothing renders.
Why do clocks show IIII instead of IV?
A clockmaking convention going back centuries, chosen for visual balance against the VIII opposite. Both are accepted on a dial.
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