Typically used to act as a comma or parenthesis to separate phrases—or even just a word—in a sentence for various reasons (i.e. an appositive).
- School is based on the three R’s—reading, writing, and ’rithemtic.
- Against all odds, Pete—the unluckiest man alive—won the lottery.
Used to connect values in a range or that are related. A good rule is to use it when you're expressing a "to" relationship.
- in years 2013–2014
- pages 10–12
- Jets won 3–2
Used to join words in a compound construction, or separate syllables of a word.
- same-sex couples
- it's pronounced hos-pi-tal-it-tee
- 4 − 2 = 2.
Typing en & em dashes on a Mac
- en-dash: option-hyphen
- em-dash: option-shift-hyphen
- en-dash: &ndash
- em-dash: &mdash
- minus sign: &minus
Source: English Language & Usage
The Elements of Typographic Style recommends the spaced en dash – like so – and argues that the length and visual magnitude of an em dash "belongs to the padded and corseted aesthetic of Victorian typography." In the United Kingdom, the spaced en dash is the house style for certain major publishers, including the Penguin Group, the Cambridge University Press, and Routledge. But this convention is not universal. The Oxford Guide to Style (2002, section 5.10.10) acknowledges that the spaced en dash is used by "other British publishers", but states that the Oxford University Press—like "most US publishers"—uses the unspaced em dash.