My English teacher told me that "dashes and commas are interchangeable".

For instance, "My friend, Alex, ran to the store." and, "My friend–Alex–ran to the store." are both grammatically correct sentences and have the same meaning.

However, is, "My friend–Alex, ran to the store." a correct sentence? Or can the sentence only have one or the other when a dependent clause is inserted into an independent clause like that?

  • In which country are you being taught this? – Kev Jun 21 '15 at 5:23
  • @Kev United States. – Henry Jun 21 '15 at 5:24
  • @Kev I understand that, what I'm asking is, can you use one comma and one dash or do you have to use both? For example, you would never use one parenthesis and one comma. – Henry Jun 21 '15 at 5:27
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    You are using hyphens (-) not dashes (—). On a Microsoft keyboard press Alt, and then 151 on the number pad (on the right) to reproduce this sign. – Mari-Lou A Jun 21 '15 at 5:34

The teacher is mistaken, though they are equivalent in your first two sample sentences.

"I can't see you -- are you here?" is grammatical, but "I can't see you, are you here?" is a comma splice.

"What the --" isn't a complete phrase, but it's acceptable in dialog. "What the," would not be. Similarly, "What the -- oh, there you are" would be acceptable in dialog, but "What the, oh, there you are" would be a mess.

The last sentence, "My friend–Alex, ran to the store," is indeed incorrect. You can set something off with em dashes or commas here, but not one of each.

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By going off the definition of interchangeable, putting emphasis on truly, no, they are not truly interchangeable. There are certain contexts when they can be, though, such as the shared example.

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