Chapter 1-2-14: Using a Noun to Modify Another Noun
In English, one noun can be placed in front of another to modify the second noun, much as a standard adjective would do. In such cases, the noun is said to become an attributive noun, also known as a noun premodifier, a noun modifier, an adjectival label, an adjectival noun, a noun adjunct, or a converted adjective. An example is "train ticket." It's possible to write "tickets for a train" instead, but attributive nouns save space.
Adjectives in English never have a distinct plural form; however, the same is not always true for attributive nouns. Normally, attributive nouns are singular, but sometimes they can be plural. Since the choice of which attributive nouns are pluralized is based on convention, this makes it tricky for second language learners to know the instances when an attributive noun being used as an adjective should be pluralized. The only way to know is through exposure to the language, and practice. Some dictionaries will also identify nouns that can be used in an attributive form.
Quirk says that "the plural attributive construction is on the increase…" (Quirk et al., A Comprehensive Grammar of the English Language, Longman, 1985, pp. 1333–1334.)
Quirk lists various situations in which the noun modifier tends to be in the plural, including situations when:
① the singular form might lead to ambiguity
• an Arts degree (a degree in the humanities); as opposed to
• an art degree (a degree in fine art)
② there is no singular form of a noun (in pluralia tantum)
• a customs officer
③ there is a need to denote variety
• a soft drinks manufacturer