When To Dash

Dash usage is even more subjective than comma usage. Authorities agree that commas, dashes, and parentheses are alternative punctuation devices for setting off mildly divorced expressions. Why one might sometimes want to set off an expression with dashes instead of commas receives opposite answers. Most often commas and dashes are said to differ in the stress they place on parenthetical matter. Material set off by dashes receives added emphasis compared to commas; or gets less, according to other authorities. The dash apparently isn't important enough for anyone to figure out.

The pair of dashes make the text they enclose parenthetical in a slightly different sense than the comma, by creating tangentiality instead of subordination. Commas set off matter that is parenthetical in the sense of being non-restrictively subordinate. A non-restrictive phrase or clause helps the reader understand the matter to which it is subordinated. While not changing the meaning of what it modifies, it adds nuance. Non-restrictive modifiers are meant to be held in the back of the mind to get a more exact understanding of the sentence's idea; expressions set off by dashes are to be noticed, filed away, and ignored. Although expressions surrounded by dashes contain matter related to the sentence, the matter doesn't refine the sentence's meaning. Dashes are interposed because the material they surround benefits by being introduced in the vicinity of the sentence's related material. Dash-surrounded matter is more parenthetical than material set off by commas but it is at the same time more central, as the sentence contains it for the tangential matter's advantage, not the sentence's. Hence the conflicted impressions that dashes give more or less emphasis than commas.

Use of parenthetical comma versus dash matter in my recent Installment in another blog illustrates the difference.

The dash: "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."

The comma: "What remains illegal under the R.A.V. standard, despite the unprotected status of frivolous filings, is apportioning the privilege of filing frivolously."

The dashes are suited to set off material so the reader can return to the main line of thought. The parenthetical material helps understand the sentence little, placement justified paragraphs ahead, where the tangentially mentioned similarity to certain speech bans is put to use.

A striking observation, apparently novel, is that dashes can always replace parenthetical commas without structural error, but commas could not properly replace the dashes in the first example. The expression set off is neither nonrestrictive clause nor nonrestrictive phrase because, not corresponding to any structural unit, it is neither clause nor phrase. An "expression," it is constituted by semantics instead of sentence structure. Many an unnecessary comma comes from an impossible wish to insert a small dash.