I saw the following sentence structure in an American textbook from 2010:
Metal A and B are much more brittle than are metal C and D.
Why is the "are" placed where it is (emphasized in bold)?
This sentence would mean the same thing without this "are", and it does not seem grammatically correct to add it here in the start of the secondary part of the sentence. If I really wanted the "are", I would have placed it in the end like this:
Metal A and B are much more brittle than metal C and D are.
What is the reason that the former sentence structure is possible?