A Beginner’s Guide to Copy-Editing: 2. Hyphens and Dashes

The hyphen - is short; the en dash – is medium; the em dash — is long; the minus sign − is similar to an en dash.
With numbers: 1-2–3—4−5 (in the order of hyphen, en dash, em dash and minus sign).

Two articles I’ve found which explain the differences between these are The hyphen, en-dash and em-dash (ndash and mdash, n-dash and m-dash) and The difference between a dash and a minus sign.

Many editors will say the correct way to use dashes when indicating a break in thought (or in the meaning of a sentence) is to use an em dash without spaces, e.g.: The boy—the one on the wall—wore a yellow cap.

I don’t like this, myself. In some fonts, the dash is long and visually distracting, and it bothers me when writing that there are no spaces. I am not the only one who reacts negatively to the em dash. You do not want the readers to be distracted from the writing because of the punctuation; the punctuation, used correctly, should be essentially invisible to the reader.

Ultimately, it’s an editorial choice (and perhaps a relic of the ages-long war between Oxford and Cambridge), but it’s increasingly normal to use space-en dash-space instead, e.g.: The boy – the one on the wall – wore a yellow cap.

So, points to bear in mind while copy-editing:

The hyphen is used to join two or more words to make a single word, and there should be no space (a line break does not count as a space) either side of the hyphen (e.g.: his mother-in-law is bi-curious). One exception to this rule is when multiple combinations are being formed from the same root-words:

The box was full of objects of multifarious colours and shapes, but all the single-coloured and -textured objects were green, and all the four- and five-cornered objects were smooth.

Another exception to this rule is where words are deliberately broken, e.g.: when discussing prefixes and suffixes:

Be consistent with spelling choices such as -ise/-isation vs -ize/-ization.

Note: 1. One common problem is that some people think you can’t or shouldn’t hyphenate more than two words, and hyphens start being dropped as a result, changing the sense of the sentence.
Note: 2. Hyphens can be used to build adjectives, but should be used with care – especially when used in a noun (consult the dictionary!).

The report on the state of the art was written using a state-of-the-art word processor.

Note: 3. Hyphens are complicated. See: Hyphenated Words: A Guide.

The hyphen is also used to break a word across two lines (but this is a job for the typesetter, not the author or editor):

The box was full of objects of multi-
farious colours and shapes.

Note: One common problem with texts that have been generated from a previously typeset medium (e.g., by scanning an old book or extracting the text from a PDF) is that the hyphens used to break words across lines get included. The above example would thus become:

The box was full of objects of multi-farious colours and shapes.

Stuttering speech (where parts of words are repeated) is punctuated using hyphens; repeated letters are capitals only if the word formally begins with a capital, e.g.: ‘I-I’d like t-to,’ I said. ‘S-so would M-Mike.’
The en dash is used to indicate ranges, e.g.: We estimate that 5–10 percent of senior citizens are engaged in May–December relationships. Bobby Green (1925–) will be talking to us tonight about his wartime experiences in Paris (1941–43).
The en dash is also used where a hyphen might be confusing, e.g.: The proposed new East Sussex–West Midlands railway failed to attract political support.
The dash when used as a break in thought or meaning (see discussion above): if using an en dash, ensure there is a space between the dash and any neighbouring words; if using an em dash, ensure there is no space between the dash and any neighbouring words.
Where the dash is used to indicate a sudden break in dialog, or a continuation of dialog after a break, there should be no spaces between the dash and the quotation marks.

‘So, there I was, minding my own business, when along comes Sally with her airs and graces—’
‘Sally Adams?’ I interrupted.
He nodded, continuing, ‘—and she dares to accuse me of intransigence!’

Where the dash is used for an aside – as an alternative to using parentheses (a handy way to avoid nesting parentheses, if nothing else) – then make sure the sentence still makes sense if the aside is removed (and, of course, this is equally necessary when using parentheses – also known as ‘round brackets’ – for an aside).
Where the dash is used for a narrative aside during dialogue, i.e., the dialog itself is not broken but the narrative intrudes, then the dash goes outside the dialog:

‘So, there I was, minding my own business, when along comes Sally with her airs and graces’—Sally Adams, I dare say—‘and she dares to accuse me of intransigence!’

If using the en dash, this will need spaces between the dashes and the quotation marks:

‘So, there I was, minding my own business, when along comes Sally with her airs and graces’ – Sally Adams, I dare say – ‘and she dares to accuse me of intransigence!’

Here are some excellent links: