Dash: Parenthetical Information
Dashes are used to offset parenthetical information – that is, information which 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 dashes (two of them), the emphasis is put on the information within the dashes.
For this usage, do make sure there are two dashes: one on each side of the parenthetical information.
While I was shopping – wandering aimlessly up and down the aisles, actually – I ran into our old neighbour.
An etymological dictionary is one of the few books – no, it’s the only book – you’ll ever need.
There has recently been an increase – though opposed fiercely, violently by many people – in alternative education practices.
He was going to call off the project – or was he? – when the client increased the payment.
Travelling – that is, travelling by public transit – can be a relaxing activity if you bring music and reading material along with you.