Skip to main content

Subject Index - Punctuation


Colons formally introduce a list or series, a question, or an amplification.

We test three types of collectors: flat plates, evacuated tubes, and parabolic troughs.

When the colon is followed by a complete sentence, capitalize the first letter of the first word after the colon.

The relationship between the two is inverse: When volume increases, pressure decreases.

Colons also often separate the parts of a ratio.

We added enough water to obtain a 3:1 dilution.

However, commas, not colons, usually follow words such as "that is," "namely," and "for example." You don't need a colon after a verb or preposition that precedes or introduces a list ("includes," "to," "with," "between," etc.). Use a colon when a noun (such as "the following") introduces a list in the text.


1. When To Use Commas

Use a comma to separate items in a series, including the next-to-last word in the series.

We develop solar thermal, wind, biomass, and photovoltaic energy technologies.

Use a comma to separate the parts of a compound sentence linked by a coordinating conjunction (such as "and," "but," "or," and "nor") when each part has its own subject and verb (unless they're very short).

I laughed at the unintentional joke, but she frowned.

Use commas to set off nonessential or nonrestrictive (parenthetical) words, phrases, and clauses from the rest of the sentence. In other words, the commas signal that the information between them is something extra and not essential to the meaning of the sentence.

The subsystem, which takes a day to install, will be delivered in two weeks.

Use commas to enclose the name of a state when it follows a city and a year when it follows the month and day.

The test systems in Gardner, Massachusetts, are performing well.
The next test sites will be in Golden, Colorado, and Chapel Hill, North Carolina.
On April 11, 1998, the committee members completed five of the six objectives.

2. When Not To Use Commas

Do not use a comma to separate compound subjects or compound verbs.

Theorists and nonspecialists alike agree on the importance of the discovery. (There is no comma between the two parts of this compound subject.)
The researchers rolled out the thin metal sheet and formed it into coils. (There is no comma between the two parts of this compound verb.)

Do not use commas to set off words or phrases that are restrictive (i.e., essential to the meaning of a sentence).

Only the sensors that were attached to the outer edge failed. (The words are essential to the meaning of the sentence.)
The system will work efficiently only if it includes storage. (The words are essential to the meaning.)

See also which and that.


Use dashes (often called "long dashes" or "em dashes") to enclose and set off parenthetical (nonessential but often illustrative) information in a sentence. Also use dashes to set off a list of items separated by commas. Do not add spaces around the dash.

The polymer components of the cell walls—cellulose, hemicellulose, and lignin—provide the feedstocks for these chemicals.

Use an em dash to signal that an important point is going to be made or that a change in the construction of the sentence follows.

The presentation concluded with a discussion of the two project factors that concern contractors the most—cost and time.

The major omission in the project assessment was the delay caused by the circuit failures—everyone knew about it, but no one mentioned it to the reviewers.

You can usually use commas, colons, and semicolons in place of dashes, but dashes add special emphasis.

Use shorter dashes known as "en dashes" (rather than a hyphen or em dash) to indicate a range or to substitute for the word "to."

25–45 cm2
2–5 runs per hour
See sections 3.1–3.6
Jan. 16–Feb. 3, 2011.

In date spans, do not use "from" in conjunction with an en dash (e.g., "from Jan. 16–Feb. 3"). The correct form is "from Jan. 16 to Feb. 3" or "Jan. 16–Feb. 3."

Do not use an en dash (or hyphen) to mean "and"; the word "between" is followed by the word "and" (not "to"): between 25 and 30.


When you want to leave out part of text material you are quoting, use ellipsis marks (three dots with a space on each side) to indicate the omission.

A participle is "a word having the characteristics of both verb and adjective ... [that] shows such verbal features as tense and voice. ..."

If the words before the ellipses form a grammatically correct sentence, put a period at the end of the sentence and follow it by ellipses. In most cases, however, you don't have to use ellipses at the beginning or end of quotes, just within them. When you add a word or words to the quote, to make it clear, enclose the added word or words in brackets to show that it is not part of the original quotation.

When you quote whole paragraphs but omit text between any two of them, center three asterisks, with spaces between them (* * *), between the paragraphs quoted. See also quotation marks.

high-performance computing

“High-performance computing” should be hyphenated. The abbreviation HPC can be used to refer to high-performance computing but should not be used to mean high-performance computer. However, the term "supercomputer" may be used in place of high-performance computer.


Use parentheses as appropriate for explanatory material in text and as shown in the examples that follow.

1. Parentheses in Equations

In equations, use parentheses, brackets, and braces in this sequence (which may be repeated as needed).

{[( )]}

2. Parentheses with Measurements

Use parentheses around English measurements that follow SI (metric) measurements.

3.1 m/s (7 mph)

3. Parentheses in Citations

When you use parentheses in text, such as for author-date references or for parenthetical (added) information, place a comma after the parentheses rather than before them.

In earlier research (Jones 1989), we showed how quantities of lipids could be increased by this method.

4. Nested Parentheses in Text

In body copy, use parentheses, brackets, and braces in this sequence, which may be repeated as needed: ([{ }]).

(The data presented here [originally derived from Mason {1998}] should not be used for location-specific analyses.)


Periods are used in some abbreviations (e.g., i.e., a.m., p.m.) and not in others (ac, dc, rpm). Most acronyms do not have periods. When you end a sentence with "etc." (although this is seldom necessary) or another abbreviation that already includes a period, do not add another one.

This paper describes the program's purpose, objectives, schedule of deliverables, etc.
(Better: This paper describes the program's purpose, objectives, and schedule of deliverables.)

quotation marks

Use quotation marks for direct quotes and the titles of articles, papers, and book chapters. In print, use "curly" or "fancy" quotation marks; on the web, use "straight" quotation marks. 

"Let's meet again in 6 months," the chairman said, "to discuss our progress."
She presented a paper titled "Materials Research in Silvered Polymer Reflectors."

Place commas (and periods) inside quotation marks; place semicolons, question marks, dashes, and exclamation points outside quotation marks unless they're part of the quotation.

"The results are in," he said.
"Can you hear me?" she asked.
Did he really say "I don't believe you"?

Use single quotation marks to indicate a quotation within material that is already enclosed in double quotation marks.

"Explain what you mean by 'confidence,'" she said.

When quotations are longer than two or three lines of text, begin them on the next line and indent them on each side (block quotations). You do not need quotation marks around block quotations, and you can use standard double quotation marks for quotes within block quotations. In in-text quotations, place reference numbers, superscripts, and author-date citations outside quotation marks (but before the final punctuation of a sentence). Place them after the final punctuation of the last sentence in a block quotation.


Semicolons indicate a stronger or more important break in the flow of words than the break indicated by a comma. Use a semicolon in compound sentences that are NOT linked by a conjunction (such as "and," "but," "or," "nor," and "yet"). Place a semicolon before conjunctive adverbs (such as "however," "hence," "therefore," "nevertheless," and "consequently") in most complex sentences containing two or more clauses. When a sentence contains items in a series, you may use a semicolon between the items if one or more of the items contains commas.

1. Using Semicolons in Compound Sentences without Conjunctions

When clauses in a sentence are closely related in meaning, a semicolon is an appropriate dividing punctuation mark. Note that the words "and," "but," "or," and "nor" do not follow semicolons.

It was difficult to reproduce the experiment; the material Smith and Jones used was not widely available. Of the 13 samples, only one did not degrade; others deteriorated an average of 8%.

2. Using Semicolons with Conjunctive Adverbs

"Yet" and "so" are usually preceded by commas in a complex sentence. But use a semicolon before such conjunctive adverbs as "then," "however," "thus," "therefore," "hence," "accordingly," "moreover," "nevertheless," "consequently," "besides," "indeed," and "subsequently"; place a comma after the adverb.

The contractor's representative was out, so I left a message.

We used the Schartz-Metterklume method in the experiment; however, the problems with this method are well known.

Energy requirements are often expressed in quads, or quadrillion Btu; therefore, this report describes the number of quads supplied annually by each option.

Use a semicolon before "i.e." ("that is") and "e.g." ("for example") and a comma after them when a clause (with a subject and verb) follows them; use a comma when a phrase or list follows.

3. Using Semicolons in a Series

When items in a series contain internal punctuation (e.g., commas) or are very long, you can separate them with semicolons. In those cases, a conjunction can follow the last semicolon.

The contaminants in the sample were TCE, 150 ppb; toluene, 220 ppb; and benzene, 265 ppb.

Promising new technologies demonstrated at the exposition included advanced wind turbines; polycrystalline, thick-film, and thin-film solar cells; fast-growing energy crops; and fuel cells.

The vendor assured us that the replacement parts, which were essential in this installation, were on order; that the parts would be delivered as soon as they arrived; and that the delay in shipment was unavoidable.

slash (solidus)

The solidus (or slash, slant, shilling mark, or virgule) is a versatile symbol that has mathematical as well as textual functions.

1. Using a Solidus in Fractions

Use a solidus to express a quotient in text when you do not need to use a displayed equation.

These structures yield photoluminescence lifetimes that are related to bulk
lifetime by the expression 1/t = 1/tB + 2 S/D.

Use a solidus in superscript and subscript fractions.


2. Using a Solidus in Text

In text, use a solidus to indicate some junctions, interfaces, and components.

gas-liquid interface
1-butyl acetate/acetic acid/water (3:1:1)

With abbreviated units of measurement, the solidus stands for "per."

2 g/cm2
355 W/m2

But spell out "per" when you spell out the units of measurement.

several cubic meters per second
a few cents per kilowatt-hour