Commas are used in many ways. One of the most common usages is to separate items in a list. Unfortunately, many of us have forgotten the rules for using commas in lists. Here's a reminder.
Use commas when writing a list of at least three words, phrases or clauses in a sentence. Place a comma between each item in the list, unless you are writing in Associated Press style. In that case, you would not place a comma before the "and" that joins the final two items. For example: "She likes chocolate, vanilla, strawberry and pistachio ice cream." Associated Press style is used in newspapers and some online content.
When writing a list of items for an academic paper, place a comma between each item in the list, including before the word "and." For example: "Germany annexed Austria, Poland, and the Netherlands." The final comma before the "and" is called the serial comma. Both the Modern Language Association and the American Psychological Association require it. Since most academic papers will be written in either MLA or APA style, students should use the serial comma.
Remember that any type of word can considered an item on a list. Nouns, verbs and adjectives all need commas when you use three or more. For example: "Delicately, gracefully, and joyfully, the ballerina twirled onto the stage."
Include a comma between each independent clause you list in a sentence. (An independent clause contains a subject and a verb and could stand on its own as a sentence because it expresses a complete thought.) For example: "She liked asparagus, he preferred spinach, and her mother liked cauliflower."
Switch to a semicolon when your list includes multi-word items, especially if those items include commas. For example: "The people who are coming are the governor, Lucy Smith; her husband, Tom Green; and the judge, Bill Thompson."