When to Use a Comma

In a series

Use a comma to separate the items in a series of three or more words, phrases or clauses.

Current job openings include positions in programming, technical writing, advertising and public relations.

Using a comma before the conjunction that joins the last two items in a series to prevent confusion regarding whether the last two items in a series are related.

In the previous example “advertising and public relations” a comma can be not used if the word “relations” refers to “advertising”. If “advertising” has nothing to do with “relations”, a comma should be used to avoid misunderstanding.

A serial comma is also known as Oxford Comma. For more information on its usage please see:

To separate independent clauses in a sentence

Use a comma to separate independent clauses that are joined by the coordinating conjunctions “and”, “but”, “yet”, “for”, “nor”, and “or.”

You do not have to back up your files, but doing so is prudent.

To separate parts of a sentence

To separate a subordinate clause or long introductory phrase at the start of a sentence from the main clause

Using a text editor, change the last line of the file.

After a dependent adverbial clause or prepositional phrase that starts a sentence

By recording transactions and automating billing, the financial software saves time and prevents costly errors.

To separate an introductory adverb from the rest of the sentence

Hopefully, he entered the personnel office.

With nonrestrictive phrases

Use a comma to set off non-defining relative clauses.

Writers often refer to this book, which is a style guide for the computer industry.

With parenthetic text

Use commas to set off short parenthetic material.

The software, with its simple interface, decreases input time by 50 percent.

In addresses

Use commas to set off components of an address when the address appears in a sentence or on one line.

Write to Plirg Systems, Inc., North Bay Village, Florida.

In dates

Use commas to separate the components of a date.

She was hired on January 1, 1996, and left six months later.

With “for example” and similar expressions

Use commas to set off expressions such as “for example,” “that is,” and “namely.”

Enter the date in MMDDYY format, for example, 110798.

In greetings

The comma should be used between the person's name and the greeting. The reason is “direct address”. We use commas to show that we are talking to the reader, not about the reader.

Hello, Rene.

Danny, thank you for your thoughtful message.

Congratulations, Michael!

I am writing to you, Kathryn, with some sad news.

I hope you know, Donelle, that we appreciate your hard work.

Exception: Do not use a comma with the greeting Dear, as in “Dear Claudio:”, “Dear Claudio,” or “Hi Claudio,”.

The “Dear Claudio” greeting is shown with both a colon and a comma. The colon is used in formal business emails. The comma is used in less formal ones.

When Not to Use a Comma

When several adjectives are used as a single modifier

This applies to adjectives that describe different qualities of an object, like its color, size, weight, etc.

Click the small black button at the top of the window.

Between two short independent clauses

Save your changes and quit the text editor.

In normal order of a sentence

If a dependent adverbial clause or prepositional phrase appears in its normal order in the sentence. This usually concerns adverbial clauses at the beginning of a sentence.

Compare these sentences “This feature saves time because it automatically updates system files.” AND “Because this feature automatically updates system files, it saves time.”

In month-year formats

Microsoft introduced Microsoft Windows version 3.0 in May 1990.