Chicago Manual of Style devotes 16 pages to the correct usage of the comma. I’m sure you’re familiar with its use with coordinate adjectives, transitional adverbs, appositive clauses, and participial phrases. I won’t insult you by repeating that information here. Instead, here are the most common errors in comma usage spotted around the Web.
Omitting a comma after city and state
When writing the name of a city followed by its state, most writers know to include the comma between the two, but often overlook the comma that’s required after the state name. Here’s how to punctuate a city, state combo correctly:
He knew he was washed up in Bath, Maine, when his show was scrubbed.
Show me the St. Louis, Missouri, map.
Omitting a comma after city and country
When writing a city name followed by the country, separate them with a comma, like this:
He got plastered in Paris, France, and broiled in London, England, last week
Omitting a comma after a month-day-year date
A complete date (consisting of a month, day, and year) requires a comma after the day and after the year.
She left the bank in Paris on July 1, 2008, for a job in Teller, Alaska.
Remember: If there’s a comma before the year, put a comma after the year.
Including a comma between a month and year
If you’re writing just a month and year (without a day), don’t separate them with a comma. And don’t include a comma after the year.
Her daughter April may return in June 2009 for the reunion.
Placing a comma outside quotation marks
Put a comma that follows a closing quotation mark inside the quote (in other words, before the ending quotation mark).
Her favorite movies are “First Wives Club,” “The Second Time Around,” and “The Last King of Scotland.”
In the U.S., commas and periods go inside the quotation marks. That’s the American way.