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I've Googled for a while now, and can't seem to find any good answers to this.

Which of the following is correct?

  1. X is the second installment in a trilogy.
  2. X is the second installment of a trilogy.
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You should say:

XXX is the second installment in a trilogy of books.

Because XXX belongs to the trilogy and is in it.

On the other hand, you'd say of a trilogy if speaking of the trilogy as a whole, like this:

XXX marks the end of a trilogy.

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+1 nice examples – speedyGonzales Feb 7 '12 at 10:09

They are both correct English. If you're speaking of the position of something in a sequence you use "of". For example,

The second of the three big snowstorms we had last year.

This usage can be extended to saying a book is the "second of a trilogy".

On the other hand, to say that a book is one of the members of a trilogy, you would say that it is "in the trilogy":

There are five books in the Hitchhiker's Guide "trilogy".

(You can't use "of" here.) So saying "second in a trilogy" is also correct.

This Google Ngram shows they are both used.

enter image description here

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As a native American English speaker, "of a trilogy" and "in a trilogy" sound essentially interchangeable to me. (Nice ngram, by the way!) – Arthaey Angosii Feb 7 '12 at 23:51

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