The Romans used these seven numerals to create all the other values. This is similar to our number system; we use 10 different values (0-9) to make all the other numbers. Roman numerals are ...

Roman numerals get nine by putting I, which equals 1, before X, which equals 10, to essentially indicated 10-1=9. For comparison, when I appears after X, as in XI, that indicates 10 + 1 ...