About every other Writing and Language class I teach, students ask me how to use the em dash ( — ), so named because it is the length of a capital M in traditional typesetting (Mac users hold down Shift + Option and press the minus key, and PC users hold down the Alt key and type 0151 on the numeric keypad). Besides the semicolon ( ; ), the em dash is the least understood and most frequently abused piece of punctuation in the English language.
If mastered, the em dash can be a versatile tool for adding emphasis and flair to an otherwise pedestrian sentence. For the purposes of the SAT, you don’t need to practice incorporating the em dash into your own writing—though you might want to—as much as you need to be able to spot correct and incorrect usages in Writing and Language passages (see what I did there?).
Em Dashes as Parentheses or Commas (Most Common)
The above sentence is an example of the most common use of the em dash on the SAT. There, two em dashes stand on either side of the parenthetical subordinate clause “though you might want to.” The em dashes set this clause aside as something of an afterthought, not essential to the meaning or grammar of the clauses surrounding it.
When a pair of em dashes is used to set off a parenthetical phrase or clause, they are interchangeable with commas and parentheses.
So, you might see that sentence written like this:
. . . you don’t need to practice how to incorporate the em dash into your own writing, though you might want to, as much as you need to be able to spot correct and incorrect usages . . .
Or like this:
. . . you don’t need to practice how to incorporate the em dash into your own writing (though you might want to) as much as you need to be able to spot correct and incorrect usages . . .
Grammatically, these sentences are identical. The only difference is emphasis (the em dashes emphasize the parenthetical clause most and the parentheses emphasize it least).
Em Dash as Semicolon
Most of my advanced students know that two dashes together set off a parenthetical, but many fall into a common trap: they think that when there is one em dash in a sentence, there always needs to be another.
They’re right that the sentence below would be wrong with only one em dash:
. . . you don’t need to practice how to incorporate the em dash into your own writing—though you might want to as much as you need to be able to spot correct and incorrect usages in Writing and Language passages. . .
That missing em dash creates a mess in the second half of the sentence. For a sentence like the one above, you also can’t mix and match punctuation—you can’t begin with an em dash and follow it up with a comma or a parenthesis, for example.
But in a sentence like the one immediately above, you only need one em dash. There, the em dash functions like a semicolon; that is, it introduces a main clause that is closely related to the preceding clause.
So, you might see that sentence read:
. . . you also can’t mix and match punctuation; you can’t begin with an em dash and follow it up with a comma or a parenthesis, for example.
Both the semicolon and the em dash are acceptable.
Em Dash as Comma
Unlike a semicolon, an em dash can also be placed between a main clause and a subordinate clause. For example, this sentence is ungrammatical:
A Lannister always pays his debts; as he should.
The semicolon here should act like a period, but the words after the semicolon do not make a complete sentence, so the semicolon above creates a fragment.
But this sentence is perfectly grammatical:
A Lannister always pays his debts—as he should.
This sentence could also read this way:
A Lannister always pays his debts, as he should.
Grammatically, the em dash and the comma have the same function here: they separate a main clause (“A Lannister always pays his debts”) and a subordinate clause (“as he should”). Again, the only difference is emphasis (the em dash adds a bit of drama to the subordinate clause).
Em Dash as Colon
Colons are used after main clauses to introduce definitions, lists, or elaborations. An em dash can be used for the same purposes. For example:
In 1886, cytologist Richard Altman used a dye technique in identifying what he then called “bioblasts,” which would later come to be known as mitochondria: the powerhouse of the cell.
Here the colon introduces a definition of the term “mitochondria.” You might see an em dash here, too:
. . . known as mitochondria—the powerhouse of the cell.
Again, the grammar of these sentences is identical: a noun phrase follows the punctuation. The difference, again, is emphasis, the em dash providing a bit more flair than the colon.
What about the Hyphen and the En Dash?
The em dash has cousins.
The hyphen ( - ) is very common. We can’t get too deep into its uses here (and anyway, its uses are not tested on the SAT, at least not yet). But you should know that hyphens link compound modifiers before nouns, such as “well-known album,” “nineteenth-century literature,” and “never-ending journey.” An exception to this rule is when the first term in the compound modifier is an –ly adverb, such as “greatly exaggerated claim” or “patiently waiting student.”
The en dash ( – ) is less common (for Mac users, hold down the option key and press the minus key). Its most common use is as a substitute for the word “to” between numbers. For example:
Read chapters 12–15 for homework.
The event will take place August 13–18.
The 2017–18 NBA season will kick off October 17.
To get into the habit of mastering dashes, keep an eye out for them when you read, and ask yourself what kind of dash you’re seeing and how it’s operating. For em dashes, you should ask: parentheses, semicolon, comma, or colon? The em dash can seem unwieldy and difficult at first, but reading and writing with them can liven up a piece of writing—and create emphasis when needed.
Stephen P. is a writer and teacher based in Los Angeles. He has taught literature and writing courses at several universities and has taught writing and reading at Elite Prep Los Angeles since 2010.