Punctuation in Parenthetical Phrases

 A parenthetical phrase is a short statement inserted into the main body of a sentence to give additional information. It is usually set apart from the rest of the sentence by means of commas, em dashes, or parentheses. Some examples:

1. John, my oldest brother, came to see me in the hospital yesterday.
2. The floor was made of stone—or at least it looked and felt like stone—and was freezing cold beneath his feet.
3. She made me pelmeni (a type of Russian dumpling) and baked potatoes for dinner.

The punctuation you should use depends on the necessity and importance of the information you’re inserting. If the information you’re adding is necessary for the reader to understand the sentence, or at least very helpful, then you’ll want to use commas. If the information is important but not actually necessary, or if it doesn’t flow too well with the rest of the sentence—as in the case of example two above—then you should generally use em dashes. If the information is extremely superfluous or disconnected from the rest of the sentence, then you’ll use parentheses.

As was the case with breaking punctuation in this post, there is often more than one possible choice when you’re deciding which punctuation to use with your parenthetical phrase. Which punctuation you use will usually depend on how strongly you want to connect the parenthetical to the rest of the sentence.

Setting apart a parenthetical phrase with em dashes is also a useful tool if the sentence already contains several commas, or if the parenthetical phrase itself contains commas. For example, this sentence:

• Whenever I went to the theater, I would always invite the group, consisting of John, Mandy, and Gus, so that I wouldn't have to sit alone.

reads a little bit more clearly like this:

• Whenever I went to the theater, I would always invite the group—consisting of John, Mandy, and Gus—so that I wouldn't have to sit alone.

Parentheses can also be used in these cases, but be wary about using them too often in fiction. Generally, you should avoid parentheses in your story unless a narrator with a strong personality is speaking—usually in either first-person or third-person omniscient perspective. Even then, parentheses should be used rarely.