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:
Inconsistency of Hyphens in Compound NounsA 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 AmbiguityYou 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).
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.