Why is "are" placed in the middle rather than the end?

Question

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?

Answer 1

Than is a particle of comparison, and it compares the clause it leads with what comes before it. In a clause that's introduced by than composed of a subject and an auxiliary verb, the subject and the auxiliary verb can switch places. This is called an inversion. In your case more specifically it is a subject-auxiliary inversion. I have included these tags in your question.

For example, you can either say:

I run much faster than he does.

or

I run much faster than does he.

However, it should also be noted that this form of inversion is becoming rarer and less commonly used these days. That's probably the reason some people are not familiar with it.