How we change what others think, feel, believe and do
The em-dash (—) and the en-dash (−) are often similar in use, where they emphatically separate out 'aside' comments that fit between phrases and which are not a direct part of the containing sentences. Depending on taste, there may or may not be spaces either side of them (the en-dash tends to use separating spaces more than the em-dash).
You can also use a single dash to separate out and hence emphasis a particular point at the end of a sentence.
The en-dash is used to connect things of equal ranking, from dates to people.
In typed text, a double-hyphen is often used instead of the em-dash--which is about the same length--as the em-dash does not appear on normal keyboards.
I am going to the pool − the one in Main Street − for a swim.
I am going to the pool—the one in Main Street—for a swim.
He is hard-working, strong — and greedy.
em- and en-dashes differ from parentheses in that the level of 'aside' is not quite as great. In spoken text, they are thus indicated by pauses, but spoken at the same volume as the main text.
Dashes give lots of visibility and hence grab the eye. This may not be desirable and so must be used with care.
The visibility adds emphasis, and the length of the dash also signals this.