Thread:Clinkzbone/@comment-4760419-20121022192028/@comment-5645141-20121022235947

Using the comma. In English, the comma is used as a thousands separator (and the period as a decimal separator), to make large numbers easier to read. So write the size of Alaska as 571,951 square miles instead of 571951 square miles. In Continental Europe the opposite is true, periods are used to separate large numbers and the comma is used for decimals. Finally, the International Systems of Units (SI) recommends that a space should be used to separate groups of three digits, and both the comma and the period should be used only to denote decimals, like $13 200,50.

From http://www.dailywritingtips.com/10-rules-for-writing-numbers-and-numerals/