When you have two complete sentences — also called independent clauses — and you connect them with a conjunction (and, but, or, for, nor, yet, so, for example), you need to insert a comma before the conjunction.
But if the second clause that makes up the sentence is a dependent clause (lacks a subject), no comma is necessary.
These are complete sentences / independent clauses that can stand alone. Each has a subject and verb:
- Tad plays guitar.
- Rachel dances.
- Toby teaches at a charter school.
These are not complete sentences, so they cannot stand alone. Each lacks a subject and depends on a preceding clause to help them make sense:
- and gives lessons to beginners
- and will perform this weekend
- and has grown fond of his students
In the examples that follow, the words in [brackets] are complete sentences / independent clauses. The words that are underlined are incomplete sentences / dependent clauses.
Do not use a comma with a dependent clause.
[Tad plays guitar] and gives lessons to beginners.
[Rachel dances] and will perform this weekend.
[Toby teaches at a charter school] and has grown fond of his students.
Do use a comma with an independent clause.
[Tad plays guitar], and [he gives lessons to beginners].
[Rachel dances], and [she will perform this weekend].
[Toby teaches at a charter school], and [he has grown fond of his students].
The following examples from reading I’ve done prompted me to write this post. Each shows the wrong (no) and then the right (yes) way to include a comma. Note that simply adding a subject to the second clause makes comma use correct.
no: [The ads are backed by significant statewide buys], and will run through the rest of July.
yes: [The ads are backed by significant statewide buys], and [they will run through and the rest of July].
no: [He doesn’t drive a sports car], and says he doesn’t intend to anytime soon.
yes: [He doesn’t drive a sports car], and [he says he doesn’t intend to anytime soon].
no: [The reality is that many of today’s working class no longer are employed in factories], and never will be again.
yes: [The reality is that many of today’s working class no longer are employed in factories], and [they never will be again].
For other examples of comma use, see my book, Grammar for People Who Hate Rules.
Chapter 29: Commas With Latin Abbreviations
Chapter 30: Commas With Academic Degrees
Chapter 42: When Does But Need a Comma?
Chapter 43: Multiple Adjectives Don’t Always Need Multiple Commas
In closing, I can’t think about commas without remembering a nickname I got early in my career. A commercial printer who helped me produce newsletters for clients called me Comma Kathy because I made, without fail, minor corrections in the proofing stage.
His intention was friendly teasing. I, of course, considered it a compliment.
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Kathy Watson
Kathy Watson has a love/hate relationship with grammar; she loves words and the punctuation that helps them make sense, yet she hates those pesky rules. A self-proclaimed ruthless editor, she prefers standard usage guidelines of The Associated Press Stylebook. Her easy-to-use Grammar for People Who Hate Rules helps people write and speak with authority and confidence. She encourages and welcomes questions and comments. (Email)