em dash

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In most of its uses the em dash (“long dash”) is a substitute for the colon, semicolon or comma, but it indicates a more emphatic or abrupt break in the sentence, or a less formal style.

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Enumerations

Use a dash, not a colon, to enclose a list of terms that does not end the sentence:

  • A number of processes—gassing, electroplating, soldering, casting, etc.—are used in the copper industry.

not

  • A number of processes: gassing, electroplating, soldering, casting, etc., are used in the copper industry.

Interruptions, pauses, afterthoughts, clarifications and emphasis

Like parentheses, a dash may be used at the end of an unfinished or interrupted statement or a pause, as in transcripts:

  • I have indicated that the appointment of the judge was terminated—or rather was not terminated but came to—

Some Hon. Members: Oh, oh!

Here the dashes are used to indicate, first, a pause and clarification and, second, an interruption.

The dash may be used to introduce an afterthought, correction or repetition:

  • Who will oppose—who are now opposed to the union?

It may similarly be used to set off an emphatic ending or one that contrasts with the remainder of the sentence:

  • To write imaginatively a man should have—imagination.

Dashes give greater emphasis to parenthetic material than do commas or parentheses. If the parenthetic material contains internal punctuation or forms a complete sentence, the commas that might have been used to enclose it should be replaced by dashes or parentheses, depending on the degree of emphasis desired or the closeness of the relationship to the rest of the sentence. Parentheses are generally used to enclose material more remote from the main thrust of the sentence, dashes for material more closely related:

  • This country is something that must be chosen—it is so easy to leave—and if we do choose it we are still choosing a violent duality.

—Margaret Atwood

The em dash is also used to attribute a quotation, as in the example above.

Summarizing

A dash is sometimes inserted before the final portion of a sentence to clarify its relationship to the rest of the sentence, often with the help of a summarizing pronoun such as all or these or with the repetition of key words:

  • Rich stores of minerals, good agricultural land, forests stretching over millions of acres, and energetic and enterprising people—all these assure Canada a bright future.

Material in apposition

Explanatory material in apposition may be set off by dashes to secure greater emphasis than would be achieved with a colon or commas or to avoid confusion with commas within the apposition:

  • Increased government funding—once hailed as a panacea for all society’s ills—is today no longer an option.

Headings

A dash may be used to separate the heading of a chapter or the like from the description of its contents or to separate subheadings within a chapter or section, as in a catalogue:

  • Gelatin Membrane Filters, White, Plain, Sartorius—A water soluble filter developed solely for…
  • Appendix A—Table of Symbols
  • ISO 2382-1994 Information-processing systems—Vocabulary—Part 14: Reliability, maintenance and availability

Lists and tables

It is sometimes used in place of bullets, numerals or letters in vertical lists:

3. Service to the public

-enquiries answered

-brochures sent out

-complaints investigated

It can represent nil or unknown in a list of figures.

Example of the use of an em dash to represent an unknown
Element Atomic weight Density Melting point
Actinium 227
Aluminum 26.98 2.7 660

Punctuation with em dash

Do not combine the dash with any mark of punctuation other than quotation marks, the question mark, the exclamation mark and occasionally the period. In particular, do not use the colon-dash (:—) to introduce a quotation or a list.

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