When people wonder where to put a comma in their writing, they often recall a teacher saying to put a comma to mark a pause. Today's comma guidelines are based on the science of sentence structure. The guidelines are similar among the academic, business and scientific writings. Writers of poetry, fiction and nonfiction use more latitude with the comma rules. While there are many comma rules, the principles of their use provide guidelines.
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Items in a Series
One of the most basic ways to use a comma is to separate items in a series of three or more items. The items can be words, phrases or clauses.
"I like apples, oranges and pears."
The three words---apples, oranges and pears---are separated with two commas.
"I like watching movies, eating popcorn and drinking sodas."
The three phrases---watching movies, eating popcorn and drinking sodas---are separated with two commas.
"Sam likes movies, Sue likes video games and Jim likes music."
The three clauses are separated with two commas.
Note that some guidelines request that an extra comma separate the items in the series. This is called the "oxford" comma, and is currently considered optional, though writers will provide the comma if a teacher or client requests it, especially for academic writing, as noted in the Purdue Online Writing Lab.
"The Constitution establishes the legislative, executive, and judicial branches of government."
Commas set off introductory elements, consisting of a word, a phrase or a clause.
"Today, we arrive in Spain."
The introductory word "Today" is followed by a comma, then the independent clause "...we arrive in Spain."
"After eating, the dog went outside."
The introductory phrase "After eating" is followed by a comma, then the independent clause "...the dog went outside." Note the misreading that could occur if there were no comma: "After eating the dog went outside."
"After the students studied, they went to the mall."
This example shows a dependent clause, "After the students studied," followed by a comma and then the independent clause.
Note the misreading that can happen without the comma in the following example:
"After we ate the dog wanted our leftovers."
In this case, the dependent clause "After we ate" needs a comma before the independent clause "...the dog wanted our leftovers."
Parenthetical and Non-Restrictive Elements
Commas enclose a word, phrase or clause that provides extra information that is not critical to the meaning of the sentence.
"My dog, Barky, loves dog treats."
Two commas surround the appositive, "Barky."
"My dog, called Barky, loves dog treats."
Two commas surround the phrase, "...called Barky."
"My dog, who is called Barky, loves dog treats."
Two commas surround the clause, "...who is called Barky." This clause is extra, or non-restrictive.
The main idea is clear: "My dog loves dog treats." The information surrounded by commas is providing extra information that is not critical to the meaning of the sentence.
Contrast this idea with the following information that is important to the meaning of the sentence, and therefore has no comma use:
"My sister Margaret got married."
In this case, the writer has three sisters, and is identifying which one married.
"My dog called Barky won the award."
If the writer has three dogs, then the lack of commas indicates which dog won the prize.
"All students who are passing the class can advance to the next level."
Consider the sentence without the dependent clause "...who are passing the class..." The sentence would then change meaning dramatically, "All students can advance to the next level." However, the dependent clause "...who are passing the class..." limits advancement only to students passing the class. Therefore, there are no commas for this restrictive, or necessary, clause.
Commas with Coordinating and Adverbial Conjunctions
Writers can place commas before a coordinating conjunction. The six coordinating conjunctions are the words "and," "but," "or," "nor," "for," "so" and "yet." A comma is placed before the coordinating conjunction that joins independent clauses.
"I like cookies, and Sue likes cake."
In this example, the two independent clauses, "I like cookies..." and "...Sue likes cake" are joined with a comma and a coordinating conjunction.
Writers can place commas after adverbial conjunctions. When a writer is joining ideas with an adverbial conjunction, also known as a conjunctive adverb, then a comma is placed before the independent clause.
"I think; therefore, I am." "I think. Therefore, I am."
In both cases, the adverbial conjunction "...therefore..." is followed by a comma and the independent clause "I am."
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