A dash is a little horizontal line that floats in the middle of a line of text (not at the bottom: that’s an underscore). It’s longer than a hyphen and is commonly used to indicate a range or a pause. Dashes are used to separate groups of words, not to separate parts of words like a hyphen does. There are three forms of dashes: em, en, and the double hyphen.
The most common types of dashes are the en dash (–) and the em dash (—). A good way to remember the difference between these two dashes is to visualize the en dash as the length of the letter N and the em dash as the length of the letter M. These dashes not only differ in length; they also serve different functions within a sentence.
Using the Em Dash to Set Off Parenthetical Information
Em dashes are often used to offset parenthetical information—that is, information you might put in parentheses. When you use parentheses, though, the emphasis is taken off the parenthetical information and put on the rest of the sentence. When you use em dashes, on the other hand, the emphasis is put on the information within the dashes.
For this usage, make sure there are two em dashes: one on each side of the parenthetical information. Using spaces before and after an em dash is a matter of preference; just be consistent. Consider the examples below for reference:
Using the Em Dash to Set Off Appositives that Contain Commas
An appositive is a small section of extra information that is inserted into a sentence for clarification. Commas are usually used to offset the appositive, but if the appositive contains one or more commas, adding additional commas would be confusing for the reader. When using an appositive that contains a comma, offset it with dashes, instead.
Using the Em Dash to Bring Focus to a List
When a sentence begins with an independent clause and ends with a list, you can use a colon between the clause and the list. When the list comes first, it’s better to use a dash to connect the list to the clause. This helps to take three potentially random things and focus them toward one idea, which is easier for the reader to process.
Using the Em Dash to Mark Sharp Turns in Thought
Dashes can act like little warnings (like yellow road signs) that whatever the writer has been doing is about to change. Dashes identify a sudden break in the sentence as the writer changes tack, perhaps going in another direction entirely, or perhaps just pausing to insert another thought. This effect is usually fairly informal, and isn’t recommended for academic writing (it might look like you don’t know what you’re talking about). Consider the examples below:
Recall that en dashes are slightly shorter in length than em dashes. En dashes may look similar to em dashes, but they function in a much different way.
Using the En Dash to Indicate Spans of Time or Ranges of Numbers
The en dash is often used to indicate spans of time or ranges of numbers. In this context, the dash should be interpreted as meaning either “to” or “through.” Consider the examples below:
Using the En Dash to Denote a Connection
The en dash may also be used to indicate a connection between two words. Use an en dash when you need to connect terms that are already hyphenated or when you are using a two-word phrase as a modifier. When the dash is used in this way, it creates a compound adjective. See the following examples: