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.