You can use an em dash, en dash or hyphen to separate items in a sentence. But which one do you use? For a while I’ve been meaning to review use of the two forms of dashes: em dash and en dash. Finally, a client asked about the use of the em dash, so I prepared this as an answer and included the hyphen, just to be thorough.
Em dash, en dash or hyphen?
There are three horizontal lines of different lengths available on the keyboard. The hyphen is shortest; the em dash is longest.
This is a hyphen –
This is an en dash –
This is an em dash—
The names of the dashes are taken from comparison with other letters in a typographer’s kit. An em dash is the width of a capital M; an en dash is the width of a capital N.
The em dash and en dash are used differently in the US and UK. Styles may vary. The information below is generally true in the US and Wikipedia says they are appropriate for APA and Chicago. You may or may not have to adhere to a specific style in your book, but you should be consistent.
A hyphen is used to join two words to form a compound (usually a compound adjective). It’s created by a key on the top row of the keyboard, second from the right. That’s the easy one.
An en dash is to indicate a range of values. Bill Clinton’s Presidency was 1992 – 2000. An en dash is normally sandwiched between between two spaces, so when you type “space-hyphen-space” in MS Word, it will convert the hyphen to an en dash, which is handy because I don’t know the key combination to make an en dash.
An em dash is used as you indicated in your question—to separate out some information. It can be used as an appositive (explanation) or for emphasis. In general, the use of the em dash to provide additional information is used when the information is surprising, or the writer wants to give it more emphasis. An em dash is not preceded or followed by a space. To type an em dash on a Macintosh keyboard, use shift-option-hyphen.
British English vs. American English
British English uses an en dash – with spaces – instead of an em dash for asides and appositives.
American English uses an em dash—without spaces—for asides and appositives.
By the way, you can also use a colon for an aside. A colon is used to expand or explain. Where I find the em dash useful is that not only does it begin the aside, it also shows the end of the aside, so it can be used mid-sentence. You can’t really do this with a colon, so only use it to expand or explain at the end of a sentence.
I looked up some of this information on Wikipedia: https://en.wikipedia.org/wiki/Dash#En_dash_versus_em_dash
Here’s another guide to the em dash. I like how the writer shows the differences in emphasis between using the em dash, parentheses and a colon. http://www.thepunctuationguide.com/em-dash.html