Appositives: Restrictive. Nonrestrictive. Commas.

If a phrase in a sentence is extra information and nonessential—in other words, if you remove it from the sentence and the sentence is still complete, correct, and doesn't change the meaning—you offset the phrase with commas.

​According to the University of Illinois Center for Writing Studies Appositive Restrictive and Nonrestrictive Clauses are defined this way:

Restrictive Clause: A restrictive modifying clause (or essential clause) is an adjective clause that is essential to the meaning of a sentence because it limits the thing it refers to. The meaning of the sentence would change if the clause were deleted. Because restrictive clauses are essential, they are not set off by commas.

• All students who do their work should pass easily.
• The car that I want is out of my price range.
• The gas company will discontinue our service unless we pay our bills by Friday.

​
Nonrestrictive Clauses: A nonrestrictive modifying clause (or nonessential clause) is an adjective clause that adds extra or nonessential information to a sentence. The meaning of the sentence would not change if the clause were to be omitted. Nonrestrictive modifying clauses are usually set off by commas.

• Edgar Allan Poe, who wrote "The Raven," is a great American poet.
• Puerto Rico was a Spanish colony until 1898, when it was ceded to the United States.

Grammar Bytes says this about Appositive Phrases:

An appositive is a noun or noun phrase that renames another noun right beside it. The appositive can be a short or long combination of words. Look at these appositive examples, all of which rename insect:

• The insect, a cockroach, is crawling across the kitchen table.
• The insect, a large cockroach, is crawling across the kitchen table.
• The insect, a large cockroach with hairy legs, is crawling across the kitchen table.
• The insect, a large, hairy-legged cockroach that has spied my bowl of oatmeal, is crawling across the kitchen table.

So, with these examples from Grammar Bytes, you know the phrase is nonrestrictive, thus needing the commas. You know this because if you removed the phrase, the sentence is still complete, correct, and makes sense: The insect is crawling across the kitchen table. 

If we look at participle phrases that come at the end of a sentence, you would use a comma like in this example:

• She yelled at me, making me cry.

The phrase 'making me cry' is extra information, thus a comma before it. 

There is a myth that says something along the lines of everywhere you 'hear' a pause, you should put a comma. Don't fall prey to the myth; however, according to Mignon Fogarty, Grammar Girl with Quick and Dirty Tips, the 'pause' can at least give you a clue as to whether or not you should consider a comma. Like in this sentence Mignon uses as another example: “He ran into the wall, banging his nose in the process.”

For a little more on comma use, check out this old TIPSY Today™ post: Commas & When to Use Them. You can also use the Search box to find other blogs on commas and all things grammar.