Always remember that the real business of the comma is just that of helping the meaning of the words and of preventing ambiguity by showing clearly the separation and connection of words and phrases. If there is possibility of misunderstanding without a comma, put one in. If the words tell their story beyond possibility of misunderstanding without a comma, there is no reason for its use. This rule will serve as a fairly dependable guide in the absence of any well recognized rule for a particular case, or where doubt exists as to the application of a rule.
Reversed, and usually in pairs, commas mark the beginning of a quotation.
In numerical statements the comma separates Arabic figures by triplets in classes of hundreds: $5,276,492.72.
In tabular work reversed commas are used as a sign for ditto.
SCHOOLS TEACHING PRINTING
Boston: Boston Typothetæ School of Printing.