I think my college newspaper experience has made me a little obsessive about writing – style, format, and clarity. My job requires reading a ton of writing done by programmers, and this group, to put it gently, is the writing equivalent of a bull in a china shop. So I post this in hopes that I can fix at least one technical writing problem with this simple rule. Always use the en-dash.
One issue I’ve been obsessing about lately is the use of dashes. Specifically, when to just hit the ‘-’ key, or (on Mac) hold down option as well to get an ‘en-dash’, or do the option-shift-dash dance to get the ‘em-dash’ (by the way I have no idea how these work on the PC, but I remember something about F-keys, aagh!). I finally decided to research the issue, and found out it’s actually very simple. At least according to Wikipedia.

The em dash, or m dash, m-rule, etc., (—), indicates a parenthetical thought—like this one—or some similar interpolation. Its name derives from its defined width of one em (originally the width of the letter m), which is the length, expressed in points, by which font sizes are typically specified.

Traditionally an em dash—like so—or a spaced em dash — like so — has been used for a dash in running text. The Elements of Typographic Style recommends the more concise 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”. The spaced en dash is also the house style for certain major publishers (Penguin, Cambridge University Press, and Routledge among them).

The en dash (always with spaces, in running text) and the spaced em dash both have a certain technical advantage over the unspaced em dash. In most typesetting and most word processing, the spacing between words is expected to be variable, so there can be full justification. Alone among punctuation that marks pauses or logical relations in text, the unspaced em dash disables this for the words between which it falls. The effect can be uneven spacing in the text.

So programmers, the dash is for hyphenation or a minus sign. Only use it as such. For a parenthetical thought, use the en-dash – option-dash on the Mac. See how nice that en-dash is?