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Why are IV and IX used for 4 and 9 in Roman numerals?

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Two vertical tablets display ancient symbols for the numbers one and ten from Egyptian hieroglyphic, Egyptian hieratic, Sumerian, and Cretan scripts, with each symbol shown in red next to its culture and approximate date. — numerals and numeral systems Examples of ancient symbols for the numbers 1 and 10.

In the Roman numeral system, the use of IV for 4 and IX for 9 is based on the subtractive principle. By placing a symbol of lesser value before one of greater value, the smaller amount is subtracted from the larger (e.g., IV = -1 + 5 = 4).12

While this principle is standard today, it was used only occasionally during the Classical period, when additive forms like IIII for 4 were often preferred in inscriptions. The Romans were generally inconsistent in their application of these rules. The subtractive method was likely adopted to simplify the representation of numbers that would otherwise require four consecutive identical symbols, as the standard practice typically limited the repetition of a symbol to three times.12