Why do the nouns in these phrases have -s?

Question

I have some questions related to these phrases

admissions policy; materials design/writing

Why do "admissions" and "materials" have -s? Shouldn't they be in the singular form because they stand before another noun/gerund?

In this article, we have "letter writing", not "letters writing"

https://www.theguardian.com/lifeandstyle/2016/nov/26/from-me-with-love-lost-art-letter-writing

How can this be explained? What is the rule here?

Answer 1

There is a "rule" which permits both singular and plural attributives (pre-head internal dependents). On pages 439 and 444 in The Cambridge Grammar of the English Language by Huddleston and Pullum (2002), it says, respectively: Pre-head complements are usually realized by nominals and Internal modifiers in pre-head position are realised by [...] nominals in plain and genitive case. Neither of the "rules" places any restrictions on inflection regarding their number. Here's a little bit on the difference between modifiers and complements.

Regarding everything else – it's up to what sounds better to English-speaking people. A rule of thumb could probably be: if it normally comes not as a single item, but as a multitude of items/phenomena/whatever, then that's how it could be denoted in writing (i.e., in the plural).

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