Chapter 3-1-1: Comma Usage (,)
When and Where to Use Commas
Before co-ordinating conjunctions joining independent clauses
Use a comma before a co-ordinating conjunction that joins independent clauses (unless the independent clauses are very short):
After introductory adverb clauses and introductory phrases
Use a comma after an introductory adverb clause and, often, after an introductory phrase (unless the phrase is very short):
Items in a series
Use a comma to separate items in a series:
Note: the comma preceding "and" is usually optional unless needed to prevent misreading. However, there are some formatting styles (such as the "Oxford" formatting style) and instructors that require you to use the comma before the conjunction "and." Ask your instructor for clarification or, to be safe, always place a comma before the word and.
Setting off parenthetical elements
Use commas to set off non-restrictive elements and other parenthetical elements. A non-restrictive modifier is a phrase or clause that does not restrict or limit the meaning of the word it is modifying. It is, in a sense, interrupting material that adds extra information to a sentence. Even though removing the non-restrictive element would result in some loss of meaning, the sentence would still make sense without it. You should usually set off non-restrictive elements with commas:
A restrictive modifier is a phrase or clause that limits the meaning of what it modifies and is essential to the basic idea expressed in the sentence. You should not set off restrictive elements with commas:
Note that you can use two other punctuation marks to set off non-restrictive elements or other parenthetical information: parentheses and dashes. Enclosing parenthetical information in parentheses reduces the importance of that information:
Placing parenthetical information between dashes has the opposite effect: it emphasises the material:
Nevertheless, you should usually set off parenthetical information with commas.
Use commas in certain date formats.
A comma is used to separate the names of days of the week, from the month, and the date from the year:
You do not need to use a comma when using a day-month-year format or if you are only providing month and a year: