History 

The comma was one of the first punctuation marks. In the 3rd century BCE, Aristophanes of Byzantium invented a system of single dots (distinctiones) that separated verses (colometry), and indicated the amount of breath needed to complete each fragment of text when reading aloud (not to comply with rules of grammar, which were not applied to punctuation marks until thousands of years later). The different lengths were signified by a dot at the bottom, middle, or top of the line. For a short passage (a komma), a media distinctio dot was placed mid-level ( · ). This is the origin of the concept of a comma, though the name came to be used for the mark itself instead of the clause it separated.

The mark used today is descended from a diagonal slash, or virgula suspensiva ( / ), used from the 13th to 17th centuries to represent a pause, notably by Aldus Manutius. In the 16th century, the virgule dropped to the bottom of the line and curved, turning into the shape used today ( , ).^[1][2][3][4]

External links: 

• Guidelines for Using Commas     English comma rules and exercises     Rules governing comma usage
• Major Comma Uses      Rules for Comma Usage      Notes on Commas
• Comma guidelines — also helpful for non-native speakers