An ELT Glossary: Adjectival Nouns

Some adjectives can also be used as nouns :

a) to denote an abstract quality - the theatre of the absurd ;   the good of the environment;  the realm of the unknown.

Adjectival nouns describing abstract qualities are grammatically singular : The unknown is scary!

b) to denote a class of people -  the rich, the poor, the unemployed, the overweight, the Left.  Nationality adjectives ending in /s/ /z/ /ʧ/ or /ʃ/ may often be used in this way : the British, the Chinese, the Danish, the Dutch, the English, the French, the Irish, the Japanese,  the Swiss, the Welsh, etc. 

Adjectival nouns describing a class of people are grammatically plural : Information on who the unemployed are—and who they are not—can provide valuable information about...

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Thornbury, S. About Language, Cambridge

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