Well, welcome back. This is the appendix on the hexadecimal number system. First of all, let's talk about why we would need to use yet another number system. The hexadecimal number system. Humans have been using the decimal number system for a long, long time. Now we have computers that use the binary number system, one more number system. It's used to help us represent the numbers that are internally represented as binary. So, if you look at this binary number with all these ones and zeros, this is not something that you would want to write down and try to figure out what this means. You wouldn't call someone up and read this number to them on the phone. They couldn't possibly write it down. So, we need a more human way of representing these binary numbers. It'll become clear to you why we've chosen the hexadecimal number system because of the conversion between binary and hexadecimal. So, the computers are working in binary and we need an easier way to look at these binary numbers, a more human way. That's why we've chosen the hexadecimal number system. Well, hexadecimal, or sometimes we'll just call this hex, is base 16. In the previous appendix, we compared base 10, the decimal number system that we've been using all our lives, and base 2, or the binary number system. In base 16, or the hex number system, we have 16 symbols to work with. Similarly, in decimal, we had 10 symbols, and in base 2, we had 2 symbols. So, there aren't 16 different digits that we can use as symbols in the hex number system. So, we borrowed letters, and we're going to use the letters A through F to represent the values 10 through 15. So, the hex number...