A pair of dash can be used to set off an interruptive word, phrase, or dependent clause within a sentence. I think it does not matter what you put between a pair of dash.
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CalifJimParenthetical remarks set off by dashes in informal personal communications (such as posts) and in journalism (especially in what we might call "Sunday Supplement Style") are perfectly acceptable. These are marks of a chatty style. They should not be used in a formal paper such as might be submitted to a scientific journal or to a similar academic organizat