A hyphen joins two or more words together (e.g. x-ray, door-to-door) while a dash separates words into parenthetical statements (e.g. She was trapped – no escape was possible).
Generally, hyphens are used to avoid confusion or ambiguity but today most words that have been hyphenated quite quickly drop the hyphen and become a single word (e.g. e-mail and email, now-a-days and nowadays). In many cases though a hyphen does make the sense clear:
- I am thinking of re-covering my sofa (to put a new cover on it)
- I would like to recover my sofa. (perhaps from someone who has borrowed it as this means ‘to get it back‘)
Hyphens and numbers
1. Use a hyphen with compound numbers from twenty-one to ninety-nine.
2. In written fractions place a hyphen between the numerator and denominator.
[Exception] if there is already a hyphen in either the numerator or the denominator, you omit the hyphen between the numerator and denominator.
- sixty-nine eighty-ninths (not ‘sixty-nine-eighty-ninths’)
- twenty-two thirty-thirds
3. Use a hyphen when the number forms part of an adjectival compund:
- France has a 35-hour working week.
- He won the 100-metre sprint.
- Charles Dickens was a great nineteenth-century novelist.
Consult your dictionary if you are not sure but remember that current usage may be more up-to-date (not uptodate) than your dictionary. There are some cases where hyphens preserve written clarity such as where there are letter collisions (co-operate, bell-like) or where a prefix is added (anti-nuclear, post-colonial), or in family relations (great-grandmother, son-in-law.)