Use a comma to separate the items of a list, adjectives of equal weight, a contrast, an introductory phrase, and two complete sentences joined by and, but, or so. A comma also prevents confusion where a reader could group the words the wrong way.
Commas matter when you want a sentence to read smoothly. If you don't know when to use one, your reader can easily get confused. Here are the most important rules for comma usage.
What is a comma?
A comma is a "soft stop": it tells the reader to pause for a moment. In long sentences, commas separate the clauses. This helps the reader understand exactly which modifiers belong to which words.
Think grammar is not important? Then read these reasons why grammar and spelling matter.
When to use a comma
Between coordinate adjectives
Commas separate words or ideas in a sentence. Use a comma between coordinate adjectives, that is, adjectives that describe the same noun with equal weight. In this sentence, the commas are used correctly:
In order to get home, we must travel over numerous winding, narrow, dangerous roads.
In this one, the comma is not used correctly:
Michael owns various red, big sweaters.
A simple test helps: if you can put and between the adjectives, or swap their order, and the sentence still sounds right, a comma can go there instead. "Winding and narrow roads" works; "red and big sweaters" does not, so the correct version is Michael owns various big red sweaters.
To show a contrast
Use a comma to set off a phrase that expresses a contrast:
It was his wealth, not his wit or charm, that first attracted her.
The kittens were adorable, but really messy.
To avoid confusion
Use a comma wherever a reader might otherwise group the words the wrong way. Compare these two sentences:
For many the season has already ended.
For many, the season has already ended.
In the second sentence, the comma separates the words correctly and keeps the reader from stumbling.
In a list of 3 or more items
Use commas to separate the items of a series of three or more things, including the last two:
She took the shirt, washed the jeans, and ironed the skirt.
To many people, the first two commas look right and the last one looks wrong. But a comma before and at the end of a list is correct, and it is especially helpful when the list is long or complex. This last comma is called the Oxford comma or the serial comma. Many writers leave it out, especially in newspapers, but that does not mean you have to.
After an introductory element
Use a comma to set off an introductory element, such as a phrase at the start of a sentence:
While taking a photo of himself, he suddenly realized how silly he looked.
While you are at it, take a look at the 4 most helpful spelling rules.
Between two independent clauses
When a short conjunction (for, and, nor, but, or, yet, or so) connects two independent clauses, that is, two parts that could each stand alone as a sentence, put a comma before it:
She hit the nail on the head, but she broke a tile on the floor.
If the second part has no subject of its own (She hit the nail on the head but broke a tile on the floor), it is not a full clause, and most style guides leave the comma out. A comma alone between two independent clauses, without the conjunction, is a mistake called a comma splice.
There are several other places where you should use a comma, and some where you should not. As your writing and your grammar improve, so will your sense of when to use a comma. Knowing the important rules for comma usage can make a real difference to the style and success of your writing. For more examples, read correct comma usage.