The serial comma (also known as the Oxford comma or Harvard comma, and sometimes referred to as the series comma) is the comma used immediately before a coordinating conjunction (usually and or or, and sometimes nor) preceding the final item in a list of three or more items. For example, a list of three countries can be punctuated as either “Portugal, Spain, and France” (with the serial comma) or as “Portugal, Spain and France” (without the serial comma).

Graphic depicting the importance of the serial comma.