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Online Writing Lab

Comma Rules Explained

When it comes to punctuation, knowing when, and when not to, use commas in writing is the biggest problem most writers face.  Correct comma usage can be hard to learn, but once it is learned, writing becomes both easier and better. 

Many writers have been told to use a comma anytime they would pause while reading a piece of writing.  While following this suggestion will lead to correct comma usage in some situations, there are many other times when following this guideline will lead to unnecessary comma usage.   Instead of using this as a guideline, there are several specific rules that dictate when commas should be used.  Learning and practicing these rules will help any writer become better at using commas. 

Rule #1: Use a comma to separate independent clauses linked with coordinating conjunctions.

If you have what can be two separate sentences but want to make them one (creating a compound sentence), use a comma and a coordinating conjunction (for, and, nor, but, or, yet, so) to link them.  (More information on compound sentences) The comma should be placed in front of the coordinating conjunction. 


Notice how the above sentences can be separated into two different sentences.  For instance, the first example could be written like this:

This is also acceptable, but if we want to connect them into one compound sentence, both a comma and a coordinating conjunction are needed to make the sentence grammatically correct.

Note: Do not place a comma before a coordinating conjunction when it is used to link words or phrases. 


Rule #2: Use a comma at the end of an introductory element.

This rule can be confusing because introductory elements are often hard to identify.  Essentially, an introductory element begins a sentence by providing a transition from the last sentence or background information before the independent clause.  More on independent clauses)  Introductory elements come in the form of prepositional phrases, subordinate clauses, and transitional expressions.  Whenever one of these is used at the beginning of a sentence, a comma should be placed after it. 


Note:  A comma is not always needed after short prepositional phrases or subordinate clauses, as long as leaving it out does not cause confusion for the reader.  However, using a comma after even a short prepositional phrase or subordinate clause is never wrong, so if in doubt, go ahead and use it.

Rule #3:  Use a comma to set off nonessential elements.

A nonessential element is a word, phrase, or clause that is not needed to complete a sentence.  In other words, it can be removed and the sentence still makes sense and is grammatically correct.  If removing the element changes the meaning of the sentence, it is essential.  Nonessential elements need to be offset with commas, both before and after. 

Examples of nonessential elements:

Rule #4:  Use a comma to separate items in a list or a series. 

A series or a list is defined as three or more.  Anytime there is a list of three or more items, use a comma to separate them.


Note:  There is often confusion about whether or not to place a comma in front of the word and in the last item of a list.  Generally, a comma should be placed in front of the and to separate the last item in the list from the one that proceeds it.  Without this comma, readers may think that the last two items are linked together in the list.  The basic rule is that when in doubt, the comma should be placed in front of the and. (This rule is often referred to as an “Oxford comma.”)

Rule #5:  Use a comma to separate multiple adjectives. 

If more than one adjective is used in a sentence, separate them with commas or by using and.  (This is also referred to as coordinate adjectives.)


Rule #6:  Use a comma to introduce a quotation.


Rule #7: Use a comma with addresses, dates, and long numbers.

When using addresses in a sentence, whether specific or not, a comma should be placed between the street and city, between the city and the state, and at the end of the address.


When using a specific date in a sentence, a comma should be placed between the day and the year and also after the year. 


When using long dates in writing, place a comma every thousandths place, or to separate numbers into groups of three, beginning on the right.


Using Commas Correctly