Let’s say, for the sake of argument, that you’re explaining something in writing — such as which punctuation marks to use to signal a break in thought — and you want to make it clear to your readers (many of whom may never have realized that there are distinctions to be made). How would you do it?
I just did.
One of three basic strategies usually suffices to set a parenthetical phrase off from its root sentence. By “parenthetical phrase,” I mean one that constitutes a digression (or a clarification) — and, despite the name, it doesn’t have to involve parentheses.
I did it again.
You see, the mildest form of parenthesis, for when you want to quickly insert a detail without distracting the reader, is a subordinate clause: a nonessential phrase framed by a pair of commas. The preceding sentence includes a subordinate clause: the one that begins “for when” and ends “the reader.” If you temporarily remove that phrase from the sentence, its structural integrity remains intact. (Try it; I’ll wait — but don’t forget to put it back when you’re done.) The subordinate-clause parenthesis is one strategy.
Another is to use the punctuation characters called parentheses. (The singular form, also used for the compositional device under discussion, is “parenthesis.”) Notice that I just inserted a parenthesis inside parentheses to make a point that isn’t key to the explanation but provides an additional dollop of information. Note also that I inserted a subordinate clause inside the parenthesis. That’s acceptable but shouldn’t be overdone, because it complicates sentence structure and can obfuscate writing.
The third device is the use of what are technically called em dashes — so called because they were originally the width of the letter “m” — though most people refer to them simply as dashes.
Digression: A pair of en dashes — the origin of this name should be apparent — are often used in place of em dashes, though they were appropriated from another use. Simple hyphens — either a single pair or two doubles — also appear in their place, especially online. Why? The hyphen is part of the ASCII roster of basic alphanumeric characters that automatically translate online, but em dashes and en dashes often have to be coded, or they’ll manifest in most browser windows as odd characters; we’ve all seen that error on even otherwise professional Web sites. Therefore, they’re used less often on the Internet. End of digression.
Dashes and their substitutes, as opposed to quotidian commas and wallflower parentheses, are best used to call attention to the inserted phrase — hey, look at me! — and note that often, they are used not in pairs with a parenthetical phrase within, but rather alone, setting a phrase off at the end of a sentence — a function commas but not parentheses share.
I have allowed these devices to proliferate here for the sake of instruction; but use them sparingly. (Especially, don’t use more than a single dash or a pair of dashes in one sentence or in consecutive sentences, or the passage may be difficult to follow.) Employed in moderation, and when each type is used according to its strength, parenthetical punctuation is a powerful textual tool — try it!