Why Roman numerals could skip zero
Roman numerals record quantities with symbols such as I, V, X, and C, whose values are combined according to rules. In CCV, for example, the two C symbols contribute 200 and V contributes 5. Nothing in the notation assigns a symbol to the tens position, so there is no empty slot that must be marked.
That is different from a place-value system such as our decimal notation. In 205, each digit’s position sets its value: the 2 means 200 because it is in the hundreds place, and the 0 preserves the empty tens place. Remove that zero and 25 means something else.
A different way to calculate
Roman numerals were useful for writing quantities, but their symbols did not make written arithmetic as straightforward as lining up digits in columns. People could calculate with counting boards or an abacus, moving counters to represent amounts; the written numeral did not have to serve as a complete calculating machine.
Medieval writers sometimes used nulla, Latin for “none,” when a calculation called for zero. But a word for absence was not the same as a standard numeral with a fixed role in a place-value system. Roman notation could express many numbers without zero because it did not depend on empty digit positions in the first place.
