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How does the additive rule work 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 additive rule functions by placing a symbol after another symbol of equal or greater value, which indicates that their values should be added together.12

For example:
* II represents $1 + 1 = 2$ .1
* LXI represents $50 + 10 + 1 = 61$ .1

This principle allows for the construction of larger numbers by sequencing symbols from left to right in descending order of value. While the Romans were historically inconsistent—sometimes preferring additive forms like IIII for 4 instead of the subtractive IV—the additive rule remains a fundamental component of how the system represents numerical values.31