Commas are your friends (or at least they should be). Excessive use of commas can be one of the easiest grammar mistakes to make. And using commas where they don't belong, or not using commas at all, can confuse your meaning beyond comprehension. To get the most out of your comma usage, keep these rules for comma usage in mind when you are writing.
Commas should be used to separate items in a series (three or more elements). Proper usage calls for inserting a comma before the "and" or "or" that precedes the last item. This will avoid confusion when using phrases such as "up and down," "peanut butter and jelly," or "rock and roll." You will find that newspapers and other periodicals will not use this serial comma, as it is termed, generally in an effort to squeeze every last bit of content into a limited space. It has become more accepted to eliminate the final comma in a series of items. Just keep in mind that this is not necessarily correct for proper grammar usage. The most important thing to keep in mind in this situation is to be consistent, always either including the final comma or eliminating this comma. Do not switch back and forth between the two usages.
The menu included French toast, cinnamon rolls, and ham and eggs.
He was late because the bus was late, he dropped his books in a puddle, and the gates were locked.
When connecting two independent clauses (effectively sentences that could stand alone), you need a coordinating conjunction (and, but, for, or, so, yet, nor) and a comma placed before this conjunction. In other words, if two (or more) parts of your sentence could stand alone as sentences without being considered sentence fragments, they should be joined by one of these conjunction words and a comma to denote the separation between the sentences. Notice that, even though in the spoken sense, there is often a pause after the coordination conjunction, in written usage, there is not a comma placed there. The one caveat to this rule is that if there are only two independent clauses that are very short and clear, there is no need for a comma.
Mary had a little lamb, so she took it to school with her.
Joe went to get a drink, Bob stopped to tie his shoe, and Jill came tumbling after both of them.
I went home and he went to work.
If you insert a comma between two independent clauses that are not joined by a corrdinating conjunction, you will have what is termed a "comma splice." This is incorrect usage and should be avoided. To correct this error, convert the two clauses to stand alone sentences, use a semi-colon, or add a coordinating conjunction.
It was raining when I went to bed, the roof was leaking again. (Incorrect)
It was raining when I went to bed. The roof was leaking again. (Correct)
It was raining when I went to bed; the roof was leaking again. (Correct)
It was raining when I went to bed, and the roof was leaking again. (Correct)
Commas should also be used to set off parenthetical elements, or those clauses and phrases which are nonessential. How do you know if it is nonessestial? Well, if it can be removed from the sentence without leaving a fragment behind and without changing the essential meaning of the sentence, it is a parenthetical element.
My aunt, who grows strawberries in her garden, brought three berry tarts to the picnic.
It is beyond him, I'm sure, to question the authority of his employer.
The girl took the apples, which were just beginning to ripen, from the basket on the table.
Commas should also be used after an introductory element in a sentence. This is a dependent clause that comes at the beginning of a sentence to clarify or emphasize that which comes after it. However, if the dependent clause comes at the end of a sentence, there is no comma before it.
Examples: Before you cross the street, be sure to look both ways.
Be sure to look both ways before you cross the street.
If there is a mild injection in the sentence (oh, my, well, etc.), there should be a comma following it.
My, you sure have grown taller over the summer!
A series of coordinate adjectives also calls for comma usage. Coordinate adjectives are adjectives that are equally important when describing a noun. Put a comma after each adjective except for the one that comes immediately before the noun. If you are unsure whether to use a comma to separate the adjectives or not, say the sentence with the word "and" in place of the comma. If it makes sense, then use the comma.
The teacher asked the unruly, disruptive students to sit down in their seats.
The little, red, rusty wagon had seen better days.
Dates, Geographic Places, Titles
Commas are need when using dates, geographic places, and some names and titles in your writing.
A comma should be used after the number denoting the day and after the number denoting the year.
She was born on January 1, 1992, in New York.
November 4, 1905, was the first day she had ever been out of the country.
Place a comma between a city and state and between a state and country. There should also be a comma after the state or country in the geographic name. The way to remember this is to think of the state or country name as being a parenthetical element and treat it as in the rule above.
We are traveling from St. Louis, Missouri, to Columbus, Ohio.
Lima, Peru, has brilliant sunshine from December to May.
A name followed by a title needs a comma between the name and the title.
Elise D. Parker, Ph.D.
Tom Cook, M.D.
Unnecessary Comma Usage
Don't use unnecessary commas. Commas should be used sparingly and only to clarify your meaning. If you are unsure about comma usage, find that you are putting commas where they do not belong, or are using them unnecessarily, it is usually best to simply rewrite the sentence.
After looking all day, Tanya told her father, he, who had lost the watch in the first place, should look for it.
Tanya told her father, who had lost the watch in the first place, that she had searched all day and he should now look for it.