If you don't know, 3,999 is MMMCMXCIX. Now, there's a rule in Roman Numerals: You can't have 4 of the same symbol together. One example is number 4. Instead of writing IIII (although that is used in some Roman numeral clocks), we can write IV. Then we subtract 1 from 5, getting us 4. Another rule is you can't have the number 0. The number system that uses 0 is the Arabic system, which is the most commonly used one today. Now using the 4/9 rule, how would we write 4,000? You can't, right? Wrong. To write 4,000, write IV and put a line over it. This indicates you're multiplying the number by 1,000. 4,000,000? Put two lines.
