Hyphens in Compound Nouns

A single noun made up of two or more words is called a compound noun (e.g., cooking-oil, pickpocket).

The rules on whether to use a hyphen, a space, or nothing between the words in a compound noun are inconsistent. In other words, some exist in all three forms (rare), some exist in two forms (common), and some exist is just one form (most common). For example:

inkwell , ink well , ink-well
iceaxe , ice axe , ice-axe
busstop , bus stop , bus-stop

Below are some guidelines on whether to hyphenate a compound noun or whether to write it as one word or two.

Overarching Guidance

guidance on hyphenating a compound noun

If you follow the process in this flowchart, your compound noun will be safe.

Use a Hyphen to Speed Up Reading

If the one-word version exists, use it. If you're unsure whether to use the two-word version or the hyphenated version, then opt for the hyphenated version. Joining the words in a compound noun with a hyphen(s) is useful to show that the words are the same grammatical entity. It speeds up reading.

Compound nouns with prepositions are nearly always hyphenated because it prevents the prepositions being read as different grammatical entities.

Use Your Spellchecker Carefully

It's a good idea to test for the one-word version with a spellchecker or a dictionary. Note that you can't test the hyphenated version or multiple-word version with a spellchecker because it will test the spelling of each word (even with the hyphenated version). In other words, if you check "pick-pocket" or "pick pocket" (which should be "pickpocket"), your spellchecker will not highlight it as an error.

More Specific Guidance on Hyphenating a Compound Noun

Here is some more specific guidance on whether to hyphenate a compound noun. Notice that none of the guidance gives a definitive rule. There are always exceptions.

Examples: cowboy, toothpaste, rainforest, sunflower, eyeball, bus stop
Guidance: Usually one word but frequently two

Examples: haircut, rainfall, snowfall, photo shoot
Guidance: Usually one word but sometimes two
preposition
Examples: passer-by, hanger-on
Guidance: Nearly always hyphenated

Examples: ballet dancing, mountain climbing, copy-editing, care-giving, bookkeeping
Guidance: Usually two words but frequently hyphenated or rarely one word

one-syllable noun

Examples: bookmaker, stocktaker, caregiver, cabdriver, winemaker, frame-maker,
Guidance: Usually one word but occasionally hyphenated

two-or-more-syllable noun

Examples: cabinet-maker, barrel-maker, potato grower, chicken farmer, pheasant plucker
Guidance: Usually hyphenated but frequently two words

[non-principal words]

Examples: daughter-in-law, jack-of-all-trades, mother-of-pearl, birds-of-prey, son-of-a-gun
Guidance: Usually hyphenated

preposition
Examples: make-up, sign-off, take-out, check-in, build-up
Guidance: Nearly always hyphenated
verb-ing (participle)

Examples: flying saucer, washing machine, swimming pool, running shoes, cooking-oil
Guidance: Nearly always two words but sometimes hyphenated to eliminate ambiguity

preposition
Examples: input, uproar, underpass, output, undercut
Guidance: Nearly always one word
preposition
Examples: underworld, underground, outpatient, afterlife, offspring
Guidance: Nearly always one word

Examples: black market, red tape, free will, full moon, blackboard, greenhouse, highway
Guidance: Usually two words but sometimes one

Use a Hyphen to Eliminate Ambiguity

You should use a hyphen to eliminate ambiguity or to prevent a reading stutter. Ambiguity or a reader stutter (when readers check back to ensure they've understood the meaning) can occur when the first word of the pairing is a substance (e.g., "water" or "ink"). For example:

water-bottle / water bottle (When the first word is a substance, a hyphen is useful to show the item is not made of that substance.)
ice-axe / ice axe (Both are acceptable, but "ice-axe" makes it clear that the axe is not made of ice.)
paper-clip / paper clip / paperclip (All 3 are acceptable. However, be aware that "paper clip" could be taken to mean a clip made of paper not a clip for paper.)
Please pass me the wire-fastener. (This is a fastener for wire not necessarily made of wire.)

Also, when the first word of the pairing ends "ing" (i.e., when it's a present participle), a hyphen helps to avoid reading stutter. For example:

changing-room / changing room (Both are acceptable, but "changing-room" makes it clear that the room is not changing.)
laughing-gas / laughing gas (Both are acceptable, but "laughing-gas" makes it clear that the gas is not laughing.)
cooking-oil / cooking oil (Both are acceptable, but "cooking-oil" makes it clear that the oil is not cooking.)

Compound Nouns as Compound Adjectives

This is a key point. If your multi-word compound noun is being used as an adjective to describe another noun, then hyphenate it (regardless of whether it is usually hyphenated or not). For example:

She likes ballet dancing. (When it's a compound noun, "ballet dancing" is two words, i.e., not hyphenated.)
She is a ballet-dancing instructor. (When it's used as an adjective, it is hyphenated.)

Here is another example:

Paint the bus stop. (When it's a compound noun, "bus stop" is two words, i.e., not hyphenated.)
Paint the bus-stop sign. (When it's used as an adjective, it is hyphenated.)