Using Hyphens - Part 2
Using Hyphens in Compound Nouns
A single noun made up of two or more words is called a compound noun (e.g., ice axe, water-bottle, inkwell). The big question with a compound noun is whether to leave it as two words (e.g., ice axe), to put a hyphen between the words (e.g., water-bottle), or to create a new word (e.g., inkwell). Unfortunately, there is no simple rule for this. You have to know how to spell the word. Occasionally, more than one version of the word is acceptable (e.g., paper clip, paper-clip, and paperclip are all acceptable). Be aware that your spellchecker will not test the two-word version or the hyphenated version as a single entity. In other words, it will not highlight air craft or air-craft as an error (even though it should be aircraft). So, you have to test the one-word version. If your spellchecker doesn't like the one-word version, you then have a choice between the two-word version and the hyphenated version. Often, this really is your choice. You should use a hyphen for clarity (i.e., to make it instantly obvious it's a single entity) and to eliminate ambiguity. For example:
pen friend and pen-friend
(The hyphenated version makes it stand out as a single entity more clearly.)
cooking oil and cooking-oil
(The hyphenated version not only makes it stand out as a single entity but also makes it clear the oil is not cooking.)
laughing gas and laughing-gas
(The hyphenated version makes it stand out as a single entity. It is also useful to show the gas is not laughing. The chance of this misunderstanding occurring is very low, but it's enough to warrant the hyphen. You're putting in the hyphen to eliminate the tiniest shred of ambiguity. If we're being honest, it's an academic exercise not a practical one.)
water bottle and water-bottle
(The hyphenated version makes it clear the bottle is not made of water. Just like in the example above, it's not a very likely misunderstanding. However, even that level of ambiguity is enough to warrant the hyphen.)
Some compound nouns, especially those with a preposition and those made up of more than two words, are best written with hyphens to show they are one entity. For example:
passer-by
hanger-on
sister-in-law
forget-me-nots
do-it-yourself