Okay, this topic will be a very simple one, it's going to be the super-important one. You just have to work as long as I had at programming to understand how valuable it is to properly comment your program. So what is a comment in the C programming language? We're in my terminal environment on the Macintosh unix environment, and let's look at a particular program that we've already may have seen. Let's look at circle.c. Here we have our comment and we're used to now writing code in which we start with a backslash and a star, and ends in a star, backslash, and then we can write whatever we want here. All of this gets discarded, so if we recall how the C compiler works. First off, there's a preprocessor, the preprocessor puts it in whatever code is necessary. There's what's called the tokenizer, and the tokenizer goes and looks through things and discards this. So this in effect becomes empty space. So why is it so important? It's important for documentation and readability. In order to be able to maintain or modify the program and understand its use, we want the program to be literate. So we could let say, add some more comments if we want, let me add some other comments here, area in kilometer, so there's a comment. Now, I'm going to show you a different common style. In original C, ANSI C for a long time, the only kind of comment was backslash, star, whatever you want star, backslash and this could be multiline. But it turns out that's inconvenient and sometimes it makes for mistakes if you misspelled this. But let's put in a rest of line comments. Notice in this case, we use two backslashes but we don't have to do anything further and only the rest of this line is considered a comment. Most compilers, unless you're using something that's probably pre 1988, will allow for this rest of line comments, let's see if my compiler will allow it. We're going to call in the compile command. No problem, now if we want we can execute, enter radius, two and that would be the area, two meters, we get an area of 12.566 square meters. So comments are critical, we can do them with the classic multi-line comment or we can use the rest of line comment, the more modern comment that tends to be less error prone and, of course, fewer characters. All right.