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Why the range extends past the usual 3,999 ceiling

Standard Roman numerals stop at 3,999 because there is no single letter standing for anything larger than M. The [msm_tool slug="roman-numerals"] extends past that using the vinculum — a bar over a letter that multiplies its value by a thousand, so a barred V represents five thousand and a barred M represents a million — which is how the range here reaches all the way to 3,999,999. The bar itself is rendered with combining overline characters, which display correctly in modern browsers and survive being copied and pasted elsewhere.

Subtractive pairs are a fixed, short list

Only six subtractive pairs are valid in the standard modern form — IV, IX, XL, XC, CD and CM — which is why four is written IV rather than IIII, and why a form like IL for forty-nine, however logical it might look by the same pattern, is not standard and is never produced here. The parser reading numerals back in applies the identical six-pair convention.

No zero, and no negative numbers

The Roman numeral system has no symbol for zero and no way to represent a negative value, a genuine structural gap rather than an oversight in this particular implementation. That absence is a real part of why positional decimal notation eventually displaced Roman numerals for actual calculation.

IIII on a clock face is not a mistake

The subtractive IV is the modern standard and what this tool produces, but the four-strokes form IIII has its own long history and still appears on clock faces by tradition, partly for the visual balance it creates against VIII on the opposite side of the dial. Both forms have genuine historical precedent; only one of them is what a converter following the modern standard should output.