How do I add commas to a number for clarity?

In large numbers, commas are used to help the reader.

A comma is placed every third digit to the left of the decimal point and so is used in numbers with four or more digits. Continue to place a comma after every third digit. For example:

• $1,000,000 (one million dollars)
• 123,456 (one hundred and twenty-three thousand, four hundred and fifty-six)