Grammar Monster
Grammar Monster

Hyphens in Compound Nouns

The Quick Answer
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 put a hyphen, a space, or nothing between the words in a compound noun are inconsistent. For example:
  • inkwell , ink well , ink-well
  • iceaxe , ice axe , ice-axe
  • waterbottle , water bottle , water-bottle
It is best to use a spellchecker or a dictionary to see what versions of the word are acceptable. There is sometimes a requirement to use a hyphen(s) to eliminate ambiguity.

Inconsistency of Hyphens in Compound Nouns

A single noun made up of two or more words is called a compound noun (e.g., water-bottle, snowman). Compound nouns written as single words (e.g., snowman, pickpocket) do not cause writers many problems. However, a compound noun written as two or more words raises the question of whether those words should be linked with a hyphen(s).

Unfortunately, there are no specific rules on forming compound nouns. For example, ink-well can be written ink well or inkwell. All three versions are acceptable.

Use a Hyphen to Eliminate Ambiguity

You should use a hyphen to eliminate ambiguity. Ambiguity is particularly prevalent when the first word of the pairing is a substance (like water or ink).

  • 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 paper of paper not a clip for paper.)

  • Please pass me the wire-fastener?
  • (This is a fastener for wire not necessarily made of wire.)
There is also potential ambiguity when the first word of the pairing ends ing (i.e., when it's a present participle.) 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.)

Not All Have a One-Word Version

Be aware that not all compound nouns have a one-word version. For example, even though inkwell and paperclip are fine, iceaxe and waterbottle are spelling mistakes. There are no rules governing this – you have to know.

Use Your Spellchecker

It's a good idea to test the one-word version with a spellchecker or a dictionary. You can't test the hyphenated version or multiple-word version with a spellchecker because it will just 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.