A dash is a mark of separation stronger than a comma, less formal than a colon, and more relaxed than parentheses — William Strunk Jr. and E. B. White: The Elements of Style. It indicates a break in the thought or structure of a sentence.
There are different conventions for spacing the dash. Leaving white space at both ends is more common — it is thought to be easier on the eye. The other style uses no white spaces at all, but writes the dash solid next to whatever goes before and follows it. Both conventions are in use:
Both parties want to reach an agreement — or so they say.
Both parties want to reach an agreement—or so they say.
How matters less than when — someone, if I’m lucky, will tell me how they want it done ;)
Dashes can be used:
· Ironically — to show a change in the construction or sentiment:
An honest politician — if such a person exists — would never agree to this plan.
· Counter-intuitively — to indicate a conclusion without expressing it:
Everyone said he was a good man but —
Goliath laughed, “They are desperate. That boy couldn’t reach higher than my kn—"
· Unexpectedly — to indicate what is not expected or an unexpected outcome:
She took a shoe box from the bottom of the pile and, lifting the lid, pulled out — an old album and a crisp bundle of yellowing letters.
· Explanatorily — instead of phrases such as: namely, that is etc:
She excelled in procrastination — blogging, tweeting and file reorganisation.
I do not leave the white space when I cut off mid-word.
I love dashes — I find them addictive — but they are a punctuation minefield.