From the new tag wiki I just made:
A noun adjunct, also called an attributive noun, is when one noun is used to give an attribute to another noun, like dog catcher, dog food, house sitter, heart surgery, running shoes, employee compensation, and Peter Principle. It is an alternative to a prepositional phrase, like food for dogs or surgery of the heart. A predicate test can be used to distinguish a noun adjunct from an adjective.
That’s just the summary. I, or someone, should get around to writing the full article some time.
By predicate test, I mean when you try to use the first word as the predicate of be. Here’s how to do that:
- running water: The water is running. Is the water running?
- running shoes: The shoes are running. Are the shoes running?
Then of course there’s running scared, which is something else again.
Notice how the first one succeeds but the second one fails. That’s because in the second case it is a gerund (a noun) not a participle (an adjective) like in the first case.
Another way to tell a noun from an adjective in this position is by stress. Adjectives are not stressed, but nouns are. With racing horse, you can do it both ways: one as an unstressed adjective (a racing horse) if the horse is racing down the path, and the other as a stressed noun (a racing horse) is the horse that’s been bred for racing.