 Here's a program that has a list of the populations of the ten largest cities in California. It uses a for loop to add them up and get a total. When we run it, we see a number with a lot of digits in it. Wouldn't it be nice if we could see the number with commas as thousand separators? Python lets us do this. To see comma separators in an integer, we'll need to add a formatting specifier. We'll put a colon and a comma to indicate we want commas and use d as the format specifier for an integer. d stands for decimal as in base 10. Now when we run the program, we get much more readable output. This also works with floating point numbers. In this program I've added a comma before the decimal point in the format specifiers for these floating point numbers. And when I run it, I get the commas where they're needed in the large number but not in numbers that are less than a thousand. They don't get a comma. That takes care of output. Let's go back to our population example. It would be nice to put commas into the population numbers to make them more readable. But if I did that, I would change this one number into three list elements, and that's certainly not what I want. Let's undo that and see how Python solves the problem. Instead of using a comma as a separator, you can use an underscore that makes the numbers somewhat more readable and everything still works fine. And here's what the program looks like after I've used the underscore as my thousand separator for all the numbers. And again, it still works great. What about user input? When we ask the user for input, they can't type commas in numbers. That'll give them an error. However, they can use underscores. Most users who aren't Python programmers won't know about the underscore trick. There is a way to let people use commas in numeric input, but you'll have to wait until we cover strings in detail to find out how. In summary, you can use underscores as thousand separators in your source code and you put a comma in your format specifiers to see thousand separators in your output.