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Teaching English: Most Common Mistakes in Comma Usage

written by: Curt Smothers • edited by: Rebecca Scudder • updated: 8/2/2012

Unlike spelling, comma punctuation rules are reliable and consistent. However, there are some comma rules that, although consistent, are not intuitive. This article focuses on some of the most common errors in comma usage caused by these “special cases.”

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    What Every Student Knows (or Should Know)

    If the period serves as a “stop sign” in a sentence, then the comma is its “traffic director.” Most students get the idea of how the comma separates items in a series, cities from states, year from month, etc. There are, however, some comma usage rules that are not intuitive. This article focuses on those quirky comma rules and gives examples.

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    Take This Comma Quiz:

    Comma What comma errors appear in the following sentences?

    1. He was born on July 14, 1952 in St. Louis.

    2. St. Louis, Missouri is located along the Mississippi River.

    3. During our trip we visited St. Louis, Missouri, Wichita, Kansas, and Denver, Colorado.

    4. My brother who is a carpenter, made the cabinet.

    5. I really dislike people, who are loud and obnoxious.

    6. While Mom was sewing the baby got into mischief.

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    Quiz Answers and Corresponding Rules

    1. Use a comma to separate the day, month, and year from the rest of the sentence:

    He was born on July 14, 1952, in St. Louis.

    2. Use a comma to separate cities and states (or other geographical pairs) from the rest of the sentence:

    St. Louis, Missouri, is located along the Mississippi River.

    3. To avoid confusion in multiple items in a series, we use a semicolon.

    During our trip we visited St. Louis, Missouri; Wichita, Kansas; and Denver, Colorado. (Some authorities -- the AP, for example -- advocate omission of the final semicolon.)

    Note: Strictly speaking, this is really a semicolon rule, but illustrates how a comma is actually "promoted" to a "flashing sign" in directing traffic in a sentence.

    4. Use commas to set off nonrestrictive relative clauses:

    My brother, who is a carpenter, made the cabinet.

    Nonrestrictive means that the sentence makes sense without what comes between the commas. In the above sentence, "My brother made the cabinet" gets along fine without the "who is a carpenter."

    5. Omit commas for restrictive relative clauses:

    I really dislike people who are loud and obnoxious.

    Restrictive means that the sentence would not make much (or the same) sense without the relative clause. In this case “who are loud and obnoxious” restricts the people being mentioned.

    6. Use commas to remove ambiguity.

    While Mom was sewing, the baby got into mischief.

    This is also an example how a comma is used to set off an introductory clause. A better example might be:

    I like peanut butter, and jelly sandwiches; or

    I like peanut butter and jelly sandwiches.

    In the first example, I like peanut butter, and I like jelly sandwiches as well. In the second, I like sandwiches made from peanut butter and jelly mixed together.