The humble hyphen

‘To hyphenate or not to hyphenate?’ – in the greater scheme of things this may seem a trivial question, but it is a recurring source of uncertainty and anxiety for editors and writers. Fowler heroically attempted to systematize its use, but it seems best to concede that this is impossible. It is often simply a matter of usage, and the best advice (when in doubt) is to consult a dictionary. Here are some examples of variations in usage: ‘water-gauge’ but ‘water heater’; water-meadow’ but ‘water main’; ‘watercolour’ (UK) but ‘water-colour’ (US).
Nevertheless, something may usefully be said.
1. It is used in compound adjectives, in particular to avoid ambiguity. There is a big difference between ‘six-inch nails’ and ‘six inch nails’; and (as Lynne Truss wittily points out) there is an even bigger difference between ‘extra-marital sex’ and ‘extra marital sex’!
2. It is used to link words, especially when used adjectivally. Compare ‘an out-of-work taxi driver’ with ‘a taxi driver who is out of work’.
3. Certain prefixes need hyphens: ‘Are you pro-abortion or pro-life?’; ‘The anti-apartheid movement was a powerful force for change.’
4. It is sometimes used to avoid ‘letter collision’, as in ‘de-ice’. However there is (in my view) no reason to hyphenate ‘coordinate’ or ‘cooperate’
5. The use of ‘floating hyphens’ is inelegant, but not incorrect. e.g. ‘Are you looking for full- or part-time employment?’
6. Hyphens are of course often used to divide words at the end of a line.