'The' article before the word plural

Question

I have seen the statements such as the following one in some literature:

"In the book 'XYZ' the word 'House' is never used in the plural."

To me the word 'THE' right before plural seems very wierd.

Would it be incorrect to write it like this?

"In the book 'XYZ' the word 'House' is never used in plural."

Why would the first one be better or correct?

Answer 1

Plural exists as both noun and adjective, and it seems to me that here it's not clear which it is. A case could be made for plural being an adjective and not a noun, the meaning being "the plural form." If so, it would be what the Cambridge Grammar of the English Language calls a fusion of functions, where the one adjective functions as both modifier and head in the noun phrase. This is fairly common and can be seen in examples like "I'll take the green/small/other." Or from The New Colossus: "Give me your tired, your poor,..."

Answer 2

"The plural" is correct, because in this case, the word "plural" is referring to the grammatical plural form, which is a thing and so it warrants a noun. From the dictionary:

noun Grammar
5. the plural number.
6. a form in the plural.

Answer 3

This is just a general reference question; any dictionary should be able to tell you that "plural" can serve as either an adjective or a noun.

plural noun 5. the plural number. 6. a form in the plural.