When you write a long number, you use commas to separate periods. Periods are simply groups of three numbers. They help make long numbers more readable. For example, here’s about as long a number as you’ll ever see:
The following list breaks this number down into periods.
This list helps you read the number. Begin all the way to the left and read, “Two hundred thirty-four quintillion, eight hundred forty-five quadrillion, twenty-one trillion, three hundred forty-nine billion, two hundred thirty million, four hundred sixty-seven thousand, three hundred four.”
When you read and write whole numbers, don’t say the word and. In math, the word and means you have a decimal point. That’s why when you write a check, you save the word and for the number of cents, which is expressed as a decimal or fraction.