These rules will help you punctuate correctly with a comma almost all the time.
A compound sentence, a sentence with two independent (main) clauses, is punctuated in three ways, the first two being most common.
- Use a comma when a coordinating conjunction connects the two main clauses.
- We went to the casino with pockets full of money , but we lost it all within an hour.
Tip: Use the acronym FANBOYS to remember the coordinating conjunctions: for, and, nor, but, or, yet, so.
- Use a semicolon (never a comma) when no coordinating conjunction is present. Another connecting word (a conjunctive adverb) may be added to help express the relationship between the clauses.
- The painting was stashed in the attic ; even the artist had forgotten about it.
- The stolen painting was hidden carefully in the attic; however, the police found it while searching the house.
Tip: Conjunctive adverbs include however, therefore, indeed, nevertheless, moreover.
- You may use a semicolon with a coordinating conjunction when the main clauses are so long and grammatically complex (containing internal punctuation) that connecting them with a comma would make the sentence hard to understand.
- She enjoyed practicing her ballet, often joined a group for square dancing, and even danced the fox trot with her father ; but most of all, she liked to tango.
An introductory element is a word, phrase, or clause that precedes the main clause of the sentence.
- It is usually set off with a comma.
- Exhausted, he fell asleep with his shoes on.
- Startled by the knock at the door, she dropped her martini.
- Because of all the publicity, he doubted that he would get a fair trial.
- A short prepositional phrase need not be set off with a comma if there is no chance of misreading.
- In 1987 she graduated from high school.
- Always use a comma if there is any chance of misreading.
- In spite of the salary , Henry made his decision to quit the job.
- In spite of the salary Henry made , his decision to quit the job was inevitable.
A nonrestrictive element is a word, phrase, or clause that gives additional information about the subject but does not limit or identify it. (This element is sometimes called a non-essential element because it may be removed without changing the essential meaning of the sentence.)
- It is set off with a comma(s) from the rest of the sentence.
- She studied harder than anyone in the class, hoping to get an A. (nonrestrictive)
- She studied harder than anyone in the class hoping to get an A. (restrictive)
An interrupter is a nonrestrictive word, phrase, or clause that comes between the subject and the verb in a sentence.
- It is set off with a pair of commas.
- Sam, the doorman, checked my ID.
- Pompeii, covered by volcanic ash, was sealed for almost 1,700 years.
- All of this exercise, you must admit, is good for you.
- A prepositional phrase that comes between the subject and verb is not set off with commas unless it is nonrestrictive (not essential to meaning).
A concluding element is a word, phrase, or subordinate (dependent) clause attached to the end of an otherwise complete sentence.
- It is set off with a comma.
- We must take the exam, prepared or not.
- He ran from the room, shrieking about a giant spider.
- He lives in Maine, even though he hates the cold weather.
Items in a series
Three or more items in a series are set off with commas.
- The garden is always sunny, warm, and fragrant.
Equal modifiers are coordinate adjectives that modify equally the same noun or pronoun. They are modifiers between which the word ‘and’ may be inserted without sounding unnatural, or modifiers which may be rearranged without changing the meaning.
- Use commas or coordinating conjunctions between them.
- We cut down the last tall, skinny fir tree on the hill.
- We cut down the last tall and skinny fir tree on the hill.
When to not use a comma
Do not use a comma:
- between two main clauses not connected with a coordinating conjunction. This creates a comma splice.
- Faulty: The tsunami hit without warning, thousands were killed.
- to separate two compound elements, such as subjects, predicates, complements, or verbs, that are joined by a coordinating conjunction.
- Faulty: He left the scene of the crime, and tried to forget it ever happened.
- before a coordinating conjunction joining two dependent clauses.
- Faulty: The contractor claimed that the housed was completed, and that the work had been done properly.
- to set off coordinating conjunctions.
- Faulty: We brought cake to the party, but, no one ate any.
- to set off restrictive elements.
- Faulty: Mrs. Abernathy’s son, Francis, is the best-behaved of her children.
- Correct: Mrs. Abernathy’s son Francis is the best-behaved of her children.