Hyphens are joiners. They join the words in a compound adjective (e.g., six-foot table, silver-service waitress), and they join the words in compound nouns (e.g., paper-clip, cooking-oil). They can also join prefixes to words (e.g., ultra-expensive, re-establish).
Their main purpose is to show the joined words are a single entity (e.g., a single adjective or a single noun). They are also useful to avoid ambiguity (e.g., a hyphen makes it clear that a paper-clip is a clip for paper and not a clip made of paper).
Rules for Using Hyphens:
Hyphens are used to join words to show they are a single entity.
- Using Hyphens in Compound Adjectives
Hyphens are used to join the words in compound adjectives. A compound adjective is a single adjective made up of more than one word.
For example (compound adjectives in bold):
- free-range eggs
- far-too-chatty individual
- eight-seater taxi
In the UK, readers will expect you to use hyphens to link the words in your compound adjectives, but, in the US, readers are more lenient. That said, regardless of which convention you're using, if you're comfortable with spotting compound adjectives, then you should group them with hyphens (or, occasionally, capital letters, italics, or quotation marks). This will:
(1) Make it easier to read.
(2) Showcase your writing skills a little.
(3) Remove the possibility for ambiguity.
When the unhyphenated version of a compound adjective is ambiguous, you must use a hyphen(s) to link its words. For example:
- a small business grant (Is this a small grant for business purposes or a grant for a small business?)
- a small-business grant (The hyphen makes it clear this is a grant for a small business. It could be a large grant.)
- a heavy water reactor (Is this a heavy reactor for use with water or a heavy-water reactor?) (Note: "heavy water" is deuterium oxide.)
- a heavy-water reactor (The hyphen makes it clear this is a deuterium-oxide reactor.)
- a fast evolving car (Is this car fast or evolving fast?)
- a fast-evolving car (The hyphen makes it clear the car is evolving quickly.)
- 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
- Using Hyphens in Prefixes
Hyphens can be used in prefixes. A prefix is a short half-word placed before another word to alter its meaning. For example (prefixes in bold):
- re-enact
- cooperate and co-operate
- antifascist and anti-fascist
The big question with a prefix is whether to use a hyphen with it or not. The guidelines on when to use a hyphen with a prefix are here. Unfortunately, the guidelines are not simple, but, often, the hyphenated and unhyphenated versions are both acceptable.
As a general rule of thumb, try to avoid using a hyphen. However, if the unhyphenated version looks too unwieldy for your taste (antiaircraft might be an example), is highlighted as spelling mistake by your spellchecker (e.g., reestablish), or is ambiguous (e.g., recover), then go for the hyphenated version.
The prefixes ex- and self- and prefixes with proper nouns (and words derived from proper nouns) are always hyphenated. For example:
- ex-army
- un-American (American is derived from the proper noun America.)