Missing Comma with Nonrestrictive Element
Just as you should not put commas around essential or restrictive information in a sentence, you must remember to put commas around information that is non-essential or nonrestrictive.
If the information is not essential to the meaning of the sentence, you need to be sure to set it off with commas. Here is an example of a sentence without proper commas around the nonrestrictive element:
Moving into a cave especially one that is already occupied is not a decision one should make without considerable forethought.
Here is the corrected version:
Moving into a cave, especially one that is already occupied, is not a decision one should make without considerable forethought.
If you have trouble deciding whether or not you need commas around “extra” information in your sentences, be sure to review the comma rules in the Punctuation area of the Excelsior OWL.