1. Commas and Nonessential (nonrestrictive) Elements
How a comma is used

2. Nonessential Phrases and Clauses
A nonessential (or nonrestrictive) phrase or clause adds information that isn’t necessary to the meaning of the sentence.
These phrases or clauses can be left out of a sentence without changing the meaning of the sentence.
Readers and writers alike use these phrases and clauses to add extra (and often times more colorful) details.

3. will set off the phrase or clause.
A comma
will set off the phrase or clause.
What does this mean?
If the nonessential element begins a sentence, a comma will follow it.
If the nonessential element is in the middle of a sentence, a comma will introduce and then follow it.
If the nonessential element ends the sentence, a comma will introduce it.

4. Nonessential Phrases
A rainbow, shimmering in the sky, suddenly appeared after the rain stopped.
The old farmhouse, built in 1903, celebrated its one hundred year of existence.
These sentences work without the phrases but lack the extra detail.
A rainbow suddenly appeared after the rain stopped.
The old farmhouse celebrated its one hundred year of existence.

5. Nonessential Clauses
Lauren, who is captain of the soccer team, is also president of the glee club.
In the 1800s, many farmers joined the National Grange Association, which helped them to gain political strength on a national level.
These sentences work without the phrases but lack the extra detail.
Lauren is also president of the glee club.
In the 1800s, many farmers joined the National Grange Association.

6. Appositives
Use commas to set off an appositive or an appositive phrase that is nonessential for the meaning of the sentence.
An appositive is a noun (and its modifiers) that add extra detail. These are considered nonessential because they are not required in the writing in order for the sentence to make sense. They simply add to it.
Does your principal, Mr. Rutten, treat students fairly?
Mr. Meehl, the mayor of Delafeld, is well known for his political aspirations.
That uniform is Mom’s favorite color, sky blue.

7. Do not…
Set off an appositive or other essential phrase or clause that answers the question
which one(s) about a word it identifies. These appositives ARE essential to the meaning of a sentence.
The dancer who won the prize was not there to accept it.
She pointed to the map that showed their route.
The students trying out for the team met after school to practice.
**A clause that begins with that is usually considered essential.

8. Parenthetical Expressions
Use commas to set off parenthetical expressions.
A parenthetical expression is a side remark that adds information. By the way, for example, however, I think, and to tell the truth are all common parenthetical expressions.
Of course, I believe that your excuse is real.
To tell you the truth, I have never told a lie in my life.
I would, for example, always tell my parents the truth.

9. are used with nonessential elements in writing to add detail.
Commas
are used with nonessential elements in writing to add detail.