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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.

?

  • @tchrist What did you mean by "don't forget the comma"? – Peter Sivák Jun 14 '13 at 20:34
  • You only have one comma for three things. You should have two of them. – tchrist Jun 14 '13 at 20:35
  • @tchrist Yes, I know because I put "and" word before the last word. Is that also informal? :) – Peter Sivák Jun 14 '13 at 20:38
  • You should still use the comma. The and is no replacement. – tchrist Jun 14 '13 at 22:34
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    Please note that what tchrist is telling you here about using a comma before and is only his opinion. There are many discussions on here about this issue and many who do not agree with him. So do not take it as a rule that you must use a comma before and. – TrevorD Jun 14 '13 at 23:19
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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."

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