I have good news about commas: there are only about nine rules governing when and where you need to use them in a sentence. I say “about nine rules” because different people divide up the rules differently. Also, some lists include technicalities like commas in dates, addresses, and numbers. Leaving dates, addresses, and numbers aside, I am going to give you nine rules about where to insert commas in sentences. If you learn and apply these rules—and only insert a comma when you can identify the rule that applies—you will get commas right almost every time.

The less-good news is that you’ll have to have a pretty good understanding of English grammar in order to apply these rules. You’ll need to know the difference between clauses and phrases, between independent clauses and dependent clauses, between compound verbs and compound clauses, between restrictive and non-restrictive elements, etc.

This week and next I will walk you through these grammatical particulars, discussing Comma Rules 1-4 today and Comma Rules 5-9 next week. Here are those rules:

Comma Rules

  1. Use a comma before the coordinating conjunction joining two independent clauses in a compound sentence.
  2. Use a comma after an introductory word, phrase, or clause that comes before the main clause.
  3. Use a comma between each of three or more items in a series.
  4. Use commas to separate two or more coordinate adjectives that modify the same noun.
  5. Use commas to set off “non-restrictive” modifying elements.
  6. Use commas to set off interrupters and parenthetical elements.
  7. Use a comma to set off nouns of direct address.
  8. Use a comma to set off verbs of attribution in dialogue.
  9. Use a comma near the end of a sentence to separate contrasted coordinate elements or to indicate a distinct pause or shift.

I realize those rules aren’t all self-explanatory. But they are comprehensive. If you can get a handle on them—and resist the temptation to stick in a comma simply because you have a vague sense that you ought to—you’ll be a comma genius.

Comma Rule 1: Compound Sentences

compound sentence consists of two independent clauses joined by a coordinating conjunction and a comma. An independent clause is a clause (that is, a group of words containing a verb and a subject) that can stand alone as a sentence. Here are two independent clauses:

  • I easily vanquished John Barber.
  • He refused to acknowledge my obvious superiority.

dependent clause also has a verb and a subject, but it cannot stand alone as a sentence. I am going to resist the temptation to get any more technical on the subject of dependent clauses. Here are two dependent clauses:

  • When I vanquished John Barber
  • Who refuses to acknowledge my obvious superiority

Either of those dependent clauses can work just fine as part of a sentence, but it would need to be attached to an independent clause. When you attach a dependent clause with an independent clause, you have a complex sentence.

But Comma Rule 1 is about compound sentences, not complex sentences. So let us return to compound sentences. As I mentioned above, in a compound sentence two independent clauses are joined by a comma and a coordinating conjunction.* The coordinating conjunctions are worth memorizing. You can remember them by the mnemonic FANBOYS: For, And, Nor, But, Or, Yet, So.

Here’s how our two independent clauses become one compound sentence:

  • I easily vanquished John Barber, but he refused to acknowledge my obvious superiority.

The comma goes before the coordinating conjunction but. Really, that’s all there is to Comma Rule 1.

If you leave out the coordinating conjunction, you have what is called a comma splice (i.e., I easily vanquished John Barber, he refused to acknowledge my obvious superiority.) If you leave out the comma, or if you leave out both the comma and the conjunction, you have what is called a run-on sentence (i.e., I easily vanquished John Barber but he refused to acknowledge my obvious superiority or I easily vanquished John Barber he refused to acknowledge my obvious superiority.) People often use the phrase run-on sentence to mean “really long sentence,” but technically a run-on is a compound sentence in which the clauses are not properly joined. A run-on can be quite short, as in I vanquished John Barber he cried.

Corollary to Rule 1: Besides compound sentences, most compounds DON’T require a comma.
Almost any grammatical element can be compounded. These compounds are most joined with and, or, either/or, or neither/nor. Here are some examples:

  • We sang and danced. (compound verb)
  • Martha and Boris got married. (compound subject)
  • We ate neither butter mints nor peanuts. (compound direct object)
  • Martha’s dress was white and lacy. (compound predicate complement)

None of those compounds require a comma. When the verb phrases in a compound verb get long, it can be tempting to insert a comma. Consider this sentence:

  • We sang many folk songs of the groom’s native land and danced the folk dances of Cincinnati.

Be strong; resist the temptation to put a comma before and. Yes, those verb phrases are long-ish, but if there’s not a second subject, it’s a compound verb, not a compound sentence. You don’t need a comma.

* the independent clauses in a compound sentence may also be joined by a semicolon with no conjunction, but that is not our concern here.

Comma Rule 2: Introductory Elements

Use a comma after an introductory word, phrase, or clause that comes before the main clause.

Here are sentences with an introductory word set off by a comma:

  • Yes, John Barber really tried to sell elevator passes in a single-story building.
  • Well, perhaps he meant no harm.
  • Still, he should be punished.

Notice that in each of the above sentences, the “introductory word” modifies the whole clause and not a specific word. That is the spirit of the “introductory word” sub-rule.

My friend Rebecca Reynolds frequently puts a comma after a coordinating conjunction at the beginning of a sentence: “Or, you could do it another way.” “But, Mr. Miyagi had a plan.” I don’t mind starting a sentence with a coordinating conjunction, but I cannot countenance that comma. I have always chalked it up to moral lassitude on Rebecca’s part. It occurs to me, however, that Rebecca could make a (weak) case that the conjunction is an introductory element and so deserves a comma.

In the following sentences, an introductory phrase is set off by a comma:

  • Having finished her supper, Cindy rose from the table.
  • To get a seat, you’d better get there early.
  • All things considered, it wasn’t such a bad riot.
  • In spite of his failings, John Barber isn’t such a bad guy.

And in these, the comma sets off an adverbial clause at the beginning of the sentence:

  • While I was doing my webinar, my dog whined to be let out.
  • If you are sick, you should go see a doctor.

One characteristic of adverbs and adverbial elements is that they are very mobile. If you want to move an adverbial clause from the beginning of a sentence to the end, English grammar allows for that. But when you do, Comma Rule 2 no longer obtains; the adverbial clause is no longer introductory, so you don’t need the comma. To wit:

  • My dog whined to be let out while I was doing my webinar.
  • You should go see a doctor if you are sick.

It might seem that if “while I was doing my webinar” or “if you are sick” is set off by a comma in one sentence, it should be set off by a comma in the other sentence. But Comma Rule 2 is concerned with the clause’s introductory position in the sentence, not with its content or even its function.

Comma Rule 3: Items in a Series

Use a comma between each of three or more items in a series. This is pretty straightforward stuff, and it applies both to series of nouns and series of other grammatical elements:

  • I like oysters, pie, and peanut butter. (series of nouns)
  • The candidate promised to lower taxes, build better roads, and institute prison reform. (series of infinitives)
  • The prosecutor argued that the guilty party was the butler, who was at the scene of the crime, who had a strong motive, and who had a pistol. (series of adjective clauses)

The attentive reader will notice that I have employed the Oxford comma in these sentences. The Oxford comma is that last comma before and at the end series. I always use the Oxford comma, but I try not to be doctrinaire about it. I’ve seen people who are so devoted to the Oxford comma that they wear t-shirts about it. I think those people would do well to ponder the truth that C.S. Lewis, the most Oxfordish person I’ve ever seen or heard tell of, didn’t use the Oxford comma in the title of The Lion, the Witch and the Wardrobe.

Comma Rule 4: Coordinate Adjectives

You have heard it said, insert a comma between two or more adjectives that modify the same noun. But I say unto you, insert a comma between two or more COORDINATE adjectives that modify the same noun. There’s a difference.

The easiest way to determine whether two adjectives are coordinate is to see what happens if you switch their order. If they are switchable, they are coordinate and therefore need a comma. Most pairs of adjectives are not coordinate. We have elaborate but unconscious systems for ordering adjectives, and it’s the exception rather than the rule when adjectives can be reordered without sounding strange. That is to say, it is the exception rather than the rule when adjectives are coordinate. I wrote about this topic here.

In the following examples, some adjective pairs are coordinate and some aren’t. They are punctuated accordingly.

  • John Barber was a sullen, difficult child.
  • He grew up in a red brick house.
  • The relentless, powerful summer sun beat down.
  • The relentless, powerful, oppressive sun beat down.

You could call John Barber a “sullen, difficult child” or a “difficult, sullen child.” You may have a preference as to the order, but nobody is going to think it strange if you switch the order. So “sullen, difficult” gets a comma. If you say he grew up in a “brick red house,” people would think it strange. So “red brick house” gets no comma. In the third example, “relentless” and “powerful” could be switched, but “summer” has to stay where it is. So there’s only one comma in “relentless, powerful summer sun.” Because “relentless,” “powerful,” and “oppressive” are all three switchable, there are two commas in the fourth example.

Next Tuesday we’ll look at Comma Rules 5-9. If you thought Rules 1-4 were great, you’re going to love Rules 5-9.

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