A manual for writers of research papers, theses, and dissertations, Ninth edition - Kate L. Turabian 2018

Commas
Punctuation
Style

Commas separate items within a sentence, including clauses, phrases, and individual words. They are especially important when a reader might mistake where a clause or phrase ends and another begins:

Before leaving the members of the committee met in the assembly room.

Before leaving, the members of the committee met in the assembly room.

For use of commas in numbers, see 23.2.2. For use of commas in citations, see 16.1.2 and 18.1.2.

21.2.1 Independent Clauses

In a sentence containing two or more independent clauses joined by a coordinating conjunction (and, but, or, nor, for, so, yet), put a comma before the conjunction. This is not a hard-and-fast rule; no comma is needed between two short independent clauses with no internal punctuation.

Students around the world want to learn English, and many young Americans are eager to teach them.

The senator arrived at noon and the president left at one.

In a sentence containing three or more short and simple independent clauses with no internal punctuation, separate the clauses with commas and add a coordinating conjunction before the last one. (Always include a comma before the coordinating conjunction.) If the clauses are longer and more complex, separate them with semicolons (see 21.3)—or, better, rewrite the sentence.

The committee designed the questionnaire, the field workers collected responses, and the statistician analyzed the results.

The committee designed the questionnaire, which was short; the field workers, who did not participate, collected responses; and the statistician analyzed the results, though not until several days later.

Ordinarily, do not insert a comma before a conjunction joining two subjects or two predicates.

The agencies that design the surveys and the analysts who evaluate the results should work together.

They do not condone such practices but attempt to refute them theoretically.

When a sentence with two independent coordinate clauses opens with a phrase or dependent clause that modifies both, put a comma after the introductory element but not between the two independent clauses.

Within ten years, interest rates surged and the housing market declined.

21.2.2 Series

In a series consisting of three or more words, phrases, or clauses with no internal punctuation of their own, separate the elements with commas. Always use a comma before the conjunction that introduces the last item.

The governor wrote his senators, the president, and the vice president.

Attending the conference were Fernandez, Sullivan, and Kendrick.

The public approved, the committee agreed, but the measure failed.

Do not use commas when all the elements in a series are joined by conjunctions.

The palette consisted of blue and green and orange.

If a series of three or more words, phrases, or clauses ends with an expression indicating continuation (and so forth, and so on, and the like, or etc.), punctuate that final expression as though it were the final item in the series. You may, however, add a comma after the continuation expression to prevent confusion after a long series.

They discussed movies, books, plays, and the like until late in the night.

Using such techniques, management can improve not only productivity but also hours, working conditions, training, benefits, and so on, without reducing wages.

Use semicolons to separate the items in a series if one or more includes commas, or if the items are long and complex (see 21.3). If such a series comes before the main verb of a sentence, however, rephrase the sentence.

The three cities that we compare are Hartford, Connecticut; Kalamazoo, Michigan; and Pasadena, California.

but not

Hartford, Connecticut; Kalamazoo, Michigan; and Pasadena, California, are the three cities that we compare.

21.2.3 Nonrestrictive Clauses and Phrases

Use paired commas to set off a nonrestrictive clause. A clause is nonrestrictive if it is not necessary to uniquely identify the noun it modifies.

These five books, which are on reserve in the library, are required reading.

Here the noun phrase These five books uniquely identifies the books that the writer has in mind; the nonrestrictive clause is not necessary to identify the books further. On the other hand, in the following sentence, the dependent clause (that are required reading) is restrictive, because it identifies a specific subset of books that are on reserve at the library. Commas are therefore not used around the clause.

The books that are required reading are on reserve in the library.

Although which is often used with restrictive clauses, American writers generally preserve the distinction between restrictive that (no comma) and nonrestrictive which (comma).

The principles delineated above apply also to restrictive and nonrestrictive phrases.

The president, wearing a red dress, attended the conference.

The woman wearing a red dress is the president.

21.2.4 Other Uses

Commas are used in a variety of other situations. (For commas in dates, see 23.3.1.)

✵ ▪ Introductory words and phrases. When you begin a sentence with an introductory element of more than a few words, follow it with a comma. A comma is not necessary after a short prepositional phrase unless the sentence could be misread without one.

If the insurrection is to succeed, the army and police must stand side by side.

Having accomplished her mission, she returned to headquarters.

To Anthony, Blake remained an enigma.

After this week the commission will be able to write its report.

✵ ▪ Two or more adjectives preceding a noun. Separate two or more adjectives preceding a noun with commas when they could, without affecting meaning, be joined by and. Do not use a comma if one or more of the adjectives is essential to (i.e., forms a unit with) the noun. (Test: if it feels awkward to change the order of the adjectives, do not use commas.)

It was a large, well-placed, beautiful house.

They strolled out into the warm, luminous night.

She refused to be identified with a traditional political label.

✵ ▪ Clarifying comments. Words and phrases such as namely, that is, and for example, which usually introduce a clarifying comment, should be followed by a comma but preceded by something stronger (such as a semicolon or a period). When you use or in the sense of “in other words,” put a comma before it. (These and similar expressions may be set off by dashes or parentheses instead; see 21.7.2 and 21.8.1.)

Many people resent accidents of fate; that is, they look on illness or bereavement as undeserved.

The compass stand, or binnacle, must be visible to the helmsman.

✵ ▪ Appositives. A word or phrase is said to be in apposition to a noun when it follows the noun and provides an explanatory equivalent for it. Nonrestrictive appositives are set off by commas; restrictive appositives are not (see 21.2.3).

Chua, a Harvard College graduate, taught at Duke for several years.

Kierkegaard, the Danish philosopher, asked, “What is anxiety?”

but

The Danish philosopher Kierkegaard asked, “What is anxiety?”

✵ ▪ Place-names. Use commas to set off multiple individual elements in names of places. (For commas in addresses, see 23.1.7.)

Cincinnati, Ohio, is on the Ohio River.

The next leg of the trip was to Florence, Italy.

✵ ▪ Interjections and conjunctive adverbs. Set off interjections, conjunctive adverbs, and the like to suggest a break in the flow of thought or the rhythm of the sentence. But omit commas when such elements do not break the continuity of the sentence.

Nevertheless, it is a matter of great importance.

It is, perhaps, the best that could be expected.

Perhaps it is therefore clear that no deposits were made.

✵ ▪ Contrasted elements. Put commas around an interjected phrase beginning with not, not only, or similar expressions. But when such a phrase consists of two components (not . . . but, not only . . . but also, and the like), commas are usually unnecessary. Use a comma between clauses of the more . . . the more type unless they are very short.

The idea, not its expression, is significant.

She was delighted with, but also disturbed by, her new freedom.

He was not only the team’s president but also a charter member.

The more it stays the same, the less it changes.

The more the merrier.

✵ ▪ Parenthetical elements. Use paired commas to set off a parenthetical element between a subject and a verb or a verb and its object. If you find yourself setting off more than one such interrupting element in a sentence, consider rewriting the sentence.

The Quinn Report was, to say the least, a bombshell.

Wolinski, after receiving instructions, left for Algiers.

✵ ▪ Repeated words. Use a comma to separate identical words. An exception is normally made for the word that.

They marched in, in twos.

Whatever is, is right.

but

She implied that that did not matter.