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In the modern usage of Roman numerals, how would a number such as 1,001,000 be converted to Roman numerals? Would it be $\overline{\mathrm M}$${\mathrm M}$ or $\overline{\mathrm {MI}}$?

Is there a set of rules governing when to use ${\mathrm M}$ and when to use $\overline{\mathrm I}$? These are the only two symbols in Roman numerals with the same value.

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This page claims that $\overline{\mathrm I}$ was not used because of M. But the historical examples given by this page suggests that something like $\overline{\mathrm{MI}}$ might have been a fine way to write it. Of note is that this Dr. Math page suggests the vertical bar/parentheses method may have preceded M and the overline, but I don't know if $\left||\mathrm I|\mathrm I\right|$ was ever an option.

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