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Consider the example

Proper nouns (such as names) and multi-word eponyms do not usually take hyphens—e.g., Josephson junction.

Would the preferred way to set off the dependent clause (the part after the em dash) be to use an em dash or a comma?
My understanding is that the em dash sets apart parenthetical phrases or clauses in a sentence. So I suppose that it sounds sound to use an em dash, or even a parenthesis.

On the other hand, correct me if I am wrong, but one couldn't use a semicolon here because the second clause is not independent?

So, we have the choice of (1) comma, (2) parenthesis, or (3) em dash.

How does one choose between these?

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    There is no second clause. << e.g., Josephson in Josephson junction >> or, as I've corrected it (the name is a two-word proper noun) << e.g., Josephson Junction >> is a parenthetical (subclass giving an example) which is set off by a dash or parentheses (a comma would be confusing as there are so many).
    – Edwin Ashworth
    Oct 4, 2016 at 22:47
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    Excellent! Thank you. In the scientific literature I find that << Josephson junction >> is spelled with lower case "junction." I suppose that makes it a non two-word proper noun.
    – Zlatko-Minev
    Oct 6, 2016 at 9:59
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    Sorry, I was reading 'Josephson Junction' as, say, a place in the US. I wouldn't call 'Josephson junction' a proper noun, unlike say 'Davis Junction'. Wikipedia does not: 'Because proper nouns are capitalized in English, the usual default for [multi-word] eponyms is to capitalize the eponymous part of a term. The common-noun part is not capitalized (unless it is part of a title or it is the first word in a sentence). For example, in Parkinson disease (named after James Parkinson), Parkinson is capitalized, but disease is not.'
    – Edwin Ashworth
    Oct 6, 2016 at 11:08

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Wikipedia covers this:

The em dash is used in several ways: primarily in places where a set of parentheses or a colon might otherwise be used, it can show an abrupt change in thought or be used where a full stop (period) is too strong and a comma too weak.

It may indicate an interpolation stronger than that demarcated by parentheses. (The degree of difference is subjective.)

  • Compare parentheses with em dashes:
    • Three alkali metals (sodium, potassium, and lithium) are the usual substituents.
    • Three alkali metals—sodium, potassium, and lithium—are the usual substituents.
  • Compare commas, em dashes and parentheses (respectively) when no internal commas intervene:
    • The food, which was delicious, reminded me of home.
    • The food—which was delicious—reminded me of home.
    • The food (which was delicious) reminded me of home.

Written dialogue can sometimes benefit from the first two (commas or dashes) in that the third (parentheses) may not suggest speech cadence to the reader as directly as the first two. But the difference is subtle and may not matter to every writer. In contrast, the third may have a subtle advantage over the first two in expository writing such as scientific writing and technical writing, because speech cadence is irrelevant there and technical readers may appreciate the grouping and nesting of phrases and clauses that brackets (and bracket nests) allow. Again, the subtlety of this distinction makes the choice a minor one.

To summarize, from weakest to strongest it's: comma ,, parenthesis (), and em dash –.

Dialogue favors commas and em dashes to help emphasize changes in tone, but parenthetical grouping has an advantage in technical writing for its nesting abilities.

It's still largely up to personal preference.

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  • I have to add that I like Edwin Ashworth's explanation in the comment above. I suppose that a comma and parenthesis would both do here, but there are already so many commas. It would be refreshing and clear to use an em dash or (). The second good point is that the second expression is not a clause, but simply a parenthetical.
    – Zlatko-Minev
    Oct 6, 2016 at 9:55

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