When used in sums, the number comma separates number classes from each other.

This use of the number comma with sums is also called the counting comma. The counting comma has only one job: it separates number classes of the decimal system from each other. You’re probably familiar with four or five of these number classes (also called number periods). These are the first six:

You’ll likely never handle numbers higher than the quadrillion class. But even if you do, the concept continues all the way to infinity: use a counting comma to separate number classes from each other.

Numbers in the thousands, for instance, use a counting comma to separate the class of units from the class of thousands:

Even when you write a number with words instead of numerals, the same rule applies; separate number classes with a counting comma:

Notice the one difference in the two examples above, however. When the counting comma is used between numerals, it doesn’t use a space after it. When used with words, however, the counting comma does use a space.

An easy way to remember when to use the counting comma is to memorize that a counting comma is always inserted after every third digit from the left of the decimal point.