Commas often play a crucial role in meaning. Even the directions for making hot cereal depend on the careful placement of a comma: Add Cream of Wheat slowly, stirring constantly. Here the comma tells the cook to add the cereal slowly. If the comma came before the word slowly, however, the cook might add the cereal all at once and stir slowly.
Because the comma can play many roles in a sentence, comma use often doesn’t follow hard and fast rules. Using commas effectively requires you to make decisions that involve audience, purpose, rhythm, and style—