Learn English – Em dash or colon in formal writing

colondashesformality

In formal writing, should I use an em dash

The Earth consists of three layers — crust, mantle, and core.

or a colon

The Earth consists of three layers: crust, mantle, and core.

Best Answer

As @tchrist said, I would recommend the colon: "The Earth consists of three layers: crust, mantle and core." However, the comma before the conjunction in the list is optional, as I understand; see http://en.wikipedia.org/wiki/Serial_comma.

From some quick research, it seems that the colon generally introduces a continuation of the thought or a list of items: more information that the writer is including. The em-dash, on the other hand, signals a change in topic (a parenthesis) or a summary of the preceding thought—a sudden movement of thought which should not be read as a continuation of the first part of the sentence.

As an example, here is a sentence from Charles Dickens' Great Expectations which uses both the colon and the em-dash:

He was gobbling mincemeat, meatbone, bread, cheese, and pork pie, all at once: staring distrustfully while he did so at the mist all round us, and often stopping—even stopping his jaws—to listen.

Notice how the sentence thought continues after the colon, while the em-dash sets apart a thought that can be omitted without damage to the original idea of the sentence: "staring distrustfully...and often stopping...to listen."

Related Solutions

Learn English – Use “you” or “one” in formal writing

Using "one" when you want to address a person in general, not specifically the reader. This was considered good practice in general. Consider this:

When the stakes are high one is often tempted to cheat.

Verses:

When the stakes are high you are often tempted to cheat.

You might prefer the former to the latter because the latter comes across as very accusatory. This is really the only circumstance I will use "one" as opposed to "you." Generally, except in very formal writing, "one" comes across as rather pretentious or old fashioned. It puts distance between the reader and writer which might be a good thing, but often isn't.

As another commentator mentioned, often it is possible to rewrite in such a way as to avoid both. For example:

When the stakes are high, cheating is more likely.

Learn English – Spaces and em-dash

If we can trust Grammar Girl's thoroughness of search, then her advice is sound:

Every style guide I checked, except the AP Stylebook, stated there should be no spaces between an em-dash and the adjacent words. That means it is a style choice. If you're writing for a newspaper, magazine, or website that uses Associated Press style, put in the spaces. Otherwise, leave them out.

So:

School is based on the three R’s—reading, writing, and ’rithemtic.

That said, em dashes are often substituted with en dashes in British English, and these are spaced when used parenthetically:

School is based on the three R’s – reading, writing, and ’rithemtic

(Also, some British English guides are more or less strict about the use of the serial comma – the above sentence may be seen as: "[...] reading, writing and ’rithemtic").

Best Answer

Related Solutions

Learn English – Use “you” or “one” in formal writing

Learn English – Spaces and em-dash

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