Dash or colon: Does the tail wag the dog?

The preceding entry concerned paired em-dashes setting off digressions. A single dash may set off a sentence-terminating digression, but in another usage, the single dash replaces a colon to introduce explanation rather than digression. Which—colon or dash—should writers favor?

Among writing academics are partisans of the colon and those of the dash, as well as neutrals. Often the criterion is register—the colon designated formal, the single dash informal—but formality doesn’t necessarily recommend usage. Legal-writing authority Professor John R. Trimble takes a distinctive position, favoring the dash over conjunctive colons because colons look overly formal (“studied”). Trimble may have over-generalized from the correct observation that the colon is overkill when the matter’s explanatory character is obvious without it, as in this sentence:

There are two parties to a sales contract—buyer and seller.

A colon would induce excessive expectations.

Another warrant for the dash in the last (italicized) example sentence is that emphasis doesn’t fall on the explanatory matter following. The colon emphasizes what follows, a pair of dashes what they enclose, but the single dash emphasizes what precedes, an emphasis writers can exploit to offset the dramatic character of what follows. This effect can trick an observer into concluding that the dash, not the meaning of what follows it, provides terminal emphasis, as here:

Employing a single em dash in a sentence commands your readers' attention, enticing them forward—c'mon, reader, let's go see what'z over here! It can also lend particular force to a terminal phrase—really it will!

Using a non-dramatic termination as paradigm, another authority correctly concluded that the single dash is backward looking, the colon forward looking: “The effect of a colon is to lead the reader forward into the following section. A dash is more like a bucket of cold water flung in the reader's face, jolting them back to the starting point of the sentence.” The perspicuous sample sentence was:

Hamlet's indecisiveness, his arrogance, his suspicion of others, his passionate, brooding, introspective nature—these all contribute to his downfall.

The misperception that the single dash emphasizes the following digression also overgeneralizes from paired dashes’ digressive emphasis. The distinction lies deep in the shape of the punctuation marks, rather than only in convention. Symmetric dashes make the enclosed matter salient, whereas a single dash makes what follows an afterthought: it looks like a tail, and everyone knows the tail doesn’t wag the dog.

A crusade against the dash?

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.