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Why isn't it "An example of mainframe operating system"?


"An example mainframe operating system is OS/390, a descendant of OS/360."

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In "an example mainframe operating system", the word "example" is like an adjective.

anonymousWhy isn't it "An example of mainframe operating system"?

This is not correct. It would have to be "an example of a mainframe operating system".

Comments  
anonymousWhy isn't it "An example of mainframe operating system"?

Yeah, the writer got a little tricky there. It is right, but you don't expect it, and it is nested deep. "Example" is used as an adjective in the sentence, it being an attributive noun. It's like "sample test questions". "Of" would be ungrammatical.

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 GPY's reply was promoted to an answer.

Thanks a lot!