Em-dashes, which emphasize digressive matter, enclose the words constituting the digression. The enclosed matter is syntactically heterogeneous, the dash being an almost unique punctuation mark, similar only to the parenthesis in its indifference to whether it ranges over descriptive modifiers, restrictive modifiers, appositives, compound predicates, propositions lacking syntactic relation to the rest of the sentence, or others. This indifference extends to the punctuation, if any, these sentence components would otherwise take.
Its syntactic indifference is probably why some writers, insecure in their knowledge of grammar, overuse the dash, and anxiety about overuse may be why some other writers are oddly averse to the dash—which, when used without restraint, can’t serve its emphasizing function. Like other vehicles of emphasis, such as bolding and italics, the dash in excess loses meaning and becomes annoyance.
The emphasis the dash imparts isn't so heavy to compel limiting its use to rare occasions, as Noreen Malone advocates in her May 24, 2011 piece in Slate’s column “The Good Word 2011: Language and how we use it.” Malone criticizes writers for substituting the dash for other punctuation marks, but in noting substitution’s prevalence, Malone unwittingly rebuts her own contention, that the dash disrupts sentence flow when interrupting it. If a sentence is unobjectionable using alternate punctuation, then the dash is innocent of fostering disruptive verbiage.
Malone observes that the dash is often used where another punctuation mark wouldn’t offend syntax. In legal writing, these alternatives are usually commas, the dash best serving legal brief writers to avoid the confusion of comma excess. (See Garner, infra.) When dashes replace commas, the matter enclosed is often a descriptive modifier, as are the five example Bryan Garner approves in The Winning Brief. (57: 231 - 233.) Writers constantly interrupt sentence flow by using descriptive modifiers, otherwise set off by commas. Without forgoing descriptive modifiers—distinguished from restrictive modifiers by interrupting sentences—the writer can’t avoid interruptions to sentence flow. Since in the sentences below flow isn't disrupted in a version using commas, the words within the dashes don’t intolerably interrupt sentence flow.
Interruption of sentence flow distinguishes descriptive modifiers—otherwise set off by commas—from restrictive modifiers.
Interruption of sentence flow distinguishes descriptive modifiers, otherwise set off by commas, from restrictive modifiers.
Less often, legal writers use dashes to set off restrictive modifiers, otherwise unpunctuated, or (below) other unpunctuated language
Unlike bans on obscenity—but like bans on speech presenting a clear and present danger of violence (Schenck v. United States (1919) 249 U.S. 47 [affirming criminal penalties for wartime military-draft-repeal agitation intended to encourage obstruction])—bans on frivolous filings are inherently viewpoint discriminatory.
Ordinarily, the conjunction of two introductory phrases need not take punctuation between the conjuncts, but here the length of the second introductory phrase and its parenthetical character recommends the dash, which also—by allowing the citation's placement beside the cited matter—avoids confusion.
Only rarely do legal brief writers use dashes to insert words—whole propositions—that they could not have added without the dash’s aid. The sole concern relevant to sentence flow is using the dash to enclose a whole proposition, with no ordinary syntactic standing within the sentence—a function of the dash that permits grammatically proper run-on sentences.
Judge Richard A. Posner provides an effective example of this usage, where a whole proposition is embedded in a sentence:
The second method, which is the pragmatic, is to determine the purpose of the rule—almost always there is a discernible purpose—and then pick the outcome that will accomplish that purpose. (How Judges Think.)
The potential harm of this form of digression is verbosity. Malone’s examples show that setting off with dashes a proposition that isn't syntactically elemental promotes inserting redundant metadiscourse. You needn’t look beyond the parody in her piece’s title: “The Case—Please Hear Me Out—Against the Em Dash: Modern prose doesn't need any more interruptions—seriously.”
Malone joins Strunk and White in her worries about replacing other punctuation with the dash. Strunk and White advise, “Use a dash only when a more common mark of punctuation seems inadequate.” The advice is off target, as the most serious potential problem of dash usage—other than overuse—occurs when the dash plays accomplice in inserting language that with other punctuation would be ungrammatical.