Today’s topic is dashes: when to use them and when not to use them. At the risk of boring you—although you are reading a post about dashes so you really have only yourself to blame—I am going to start with the distinction between hyphens and dashes. A hyphen joins two words into a unit (e.g., ‘on-site manager’ or ‘lotus-eater’) or indicates a word break at the end of a line of text. A short dash (often called an en-dash because it is the width of the letter ‘n’) separates items such as dates or page numbers (e.g., 1711–1776 or pp. 106–7). A long dash (often called an em-dash because it is the width of a letter ‘m’) is what we will be talking about below. A hyphen is produced by a simple keystroke on your keyboard; dashes are symbols that can be inserted into a document in a variety of ways. For more on hyphens and dashes, you can consult the online Chicago Manual of Style.
Just in case anyone is still reading, I am now going to talk about using dashes in your writing. First, we will look at single dashes, which I will argue are insufficiently formal for academic writing, although some would disagree. Here is an example:
Recombinant DNA technology has left an indelible mark on the basic ingredient of human survival—food production.
This dash could be replaced by either a comma or a colon. I would suggest a colon because a colon signals unambiguously that what follows will complement or complete what precedes it. A comma would work, but since commas play so many roles in our writing, I generally choose a colon over a comma when they go head-to-head. Here is another example:
The range of Hume’s writings help us to understand his reservations about a revolutionary approach—such an approach would be unlikely for a thinker so immersed in historical considerations.
In this case, you could replace the dash with either a semicolon or a period. Generally, a semicolon will work best since it will provide the closeness that the author was trying to convey with the dash.
I am only arguing against the single dash in academic writing—it is absolutely fine in a more casual context like, say, a blog post. My reason for this advice is that the dash can play the role of a comma, colon, or period without doing anything unique. Nothing special is added and something may be lost. You may lose clarity, and you will certainly lose formality. I am being, of course, overly opinionated; if you wish to defend the use of single dashes in formal writing, I hope you’ll do so in the comments below. One possible line of defence is certainly the drama of the single dash; since it breaks up a sentence in an unconventional manner, it does draw real attention to what follows it. Now let’s turn to the use of double dashes.
Double dashes are used instead of commas (or parentheses) to interrupt a sentence. The phrase separated by dashes must be grammatically inessential, by which I simply mean that the sentence will still work without that phrase. Here is an example:
The issues that confront students entering late-stream French immersion programs—most importantly, their lack of familiarity with instructional vocabularies—are a pressing concern for school administrators.
It is often said that we use dashes to signal that something crucial is being added to a sentence and use parentheses to signal that the interruption is relatively unimportant (e.g., to give dates or citations or examples). In this schema, commas fall somewhere in the middle. There is some truth to this division, but I find the next three principles more helpful in deciding whether to use double dashes:
1. Most importantly, double dashes can be used to add an element to a sentence that already has its share of commas.
2. Double dashes are very common in some fields (most humanities fields, for instance) and rare in others (in many scientific fields, for instance). If they are rare in your field, it is unlikely that you would want to use them.
3. Regardless of field, double dashes are a stylistic variant and should be used sparingly; they draw attention to themselves, and the audience will grow tired of them if they are overused.
Overall, I suggest using double dashes—if they are appropriate to your field of study—in moderation and replacing single dashes with colons, commas, or even periods, if only in your formal writing.
Note: For more on dashes, see Interrupting Yourself.