In the Roman numeral system, a system that uses letters to represent numerals, I is the equivalent of one. Two is written as II, and three is equal to III. A letter cannot appear more than three times in succession.
Other basic units in the Roman numeral system include V for five, X for 10, L for 50, C for 100, D for 500 and M for 1,000. Writing the number four involves portraying it as five minus one, or IV. When a letter representing a smaller number than the one to its immediate right, it is subtracted from the larger number. Likewise, IX equals 10 minus one, or nine.