Rules, Dashes and the Punctuation of Parenthetical Clauses

Using En Rules and Em Rules To Punctuate Parenthetical Clauses
An em rule or em dash, which is about as wide as an uppercase M, is longer than an en rule or en dash, which is about the width of an uppercase N, but both forms can be used to punctuate parenthetical clauses. Both should not be used, however, for the same purpose within a single document, so it is important to check publisher guidelines to see if one form of rule or the other is preferred for parenthetical clauses. If so, that form should be used consistently; if not, the author can use whichever dash she or he prefers as long as the usage is consistent throughout a single document.

When an en rule is used to punctuate a parenthetical clause, a space should be added on each side of the rule. If the parenthetical clause appears in the middle of a sentence, spaced en rules should be used to surround the clause, as they are in this sentence: ‘I just received a letter from a journal – a top-tier one! – telling me that my paper has been accepted for publication.’ When, on the other hand, the parenthetical clause appears at the end of a sentence, only the opening en rule is required, so a closing en rule should not be used: ‘I just received some wonderful news about the research paper I submitted for publication – it has been accepted by a top tier journal!’
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Em rules are used almost exactly as en rules are to punctuate parenthetical clauses, but when em rules are used for this purpose no spaces should appear around them. Each em rule should therefore be closed up whether the parenthetical clause appears in the middle of a sentence or at the end. ‘I just received a letter from a journal—a top-tier one!—telling me that my paper has been accepted for publication’ shows the correct format for the first, while ‘I just received some wonderful news about the research paper I submitted for publication—it has been accepted by a top tier journal!’ demonstrates the second.

Whether you are using en rules or em rules to punctuate parenthetical clauses, a few conventions should be kept in mind. Either type of rule indicates a more pronounced break in a sentence than commas would and also highlights the parenthetical clause more than parentheses would. When a dash of either kind is used to mark a clause at the end of a sentence, it functions rather like a colon, but is somewhat less formal and often expresses an aside or afterthought. No punctuation should precede an opening parenthetical en rule or em rule, and while the closing rule of a parenthetical clause can be preceded by a question or exclamation mark (as in my examples for mid-sentence parenthetical clauses above), it should never be preceded by a comma, semicolon, colon or full stop. Finally, although using en rules and em rules for punctuating parenthetical clauses is acceptable in formal prose, excessive use of either kind of dash for marking parenthetical material (in every sentence, for instance) is poor style, and as a general rule no more than one parenthetical clause marked by dashes should be used in a single sentence. It is therefore a good idea to vary your usage, employing commas, parentheses or dashes as appropriate depending on the context and significance of the parenthetical material you are punctuating.

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