The Writing Center @ The University of Wisconsin - Madison
The Writer's Handbook
An Editing Checklist

7. Omitted commas

Use commas to signal nonrestrictive or nonessential material, to prevent confusion, and to indicate relationships among ideas and sentence parts.

Incorrect:

When it comes to eating people differ in their tastes.

Correct:

When it comes to eating, people differ in their tastes.

Incorrect:

The Huns who were Mongolian invaded Gaul in 451.

Correct:

The Huns, who were Mongolian, invaded Gaul in 451.

["Who were Mongolian" adds information but does not change the core meaning of the sentence because Huns were a Mongolian people; this material is therefore nonrestrictive or nonessential.]

For more information on commas see Commas: Punctuating Restrictive and Non-restrictive Modifiers and Punctuating Coordinating Conjunctions and Sentence Adverbs, or take one of the free grammar, style, and style classes offered by the Writing Center.