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).

Examples:
  • 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.)

See Also

Forming the plurals of compound nouns What are nouns? The different types of nouns Alternatives to hyphens in compound adjectives Hyphens in compound adjectives Hyphens in prefixes