Grammar & Punctuation – Dash – English editing.

A punctuation mark that looks like an extended hyphen. It comes in two sizes, an em dash (-) and an en dash (-).
An em dash is used to mark a break in sentences:
 It can be used in pairs to show words in Parenthesis:
In brute material terms he was an accomplice – in fact, a conspirator – to the murder of millions of children.
 It can introduce something that develops, or is an example of, what has gone before:
You must have seen it, I am sure – the blue flag with a white square in the middle.
 In more formal writing, a colon would be used instead of a dash.
 It can introduce an aside by the writer:
I occupied Piers’ old studio and Toby the three guest rooms – this purely for company.
 In direct speech it can show that someone breaks off in mid sentence, or is interrupted:
I smiled and she said, ‘You mean you want me to – ?
An en dash is used to show sequences:
1999-2000
An A-Z guide.
In sequences either use from 1999 to 2000 or 1999-2000; mixing the two styles, e.g. from 1999-2000, is wrong.
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