What grammatical concept is used here? (“A that would have p.p. had B p.p.”)

Question

The following is an excerpt from RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1:

§14.13 Content-Length

The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

. And this is my first time seeing this structure of a noun phrase:

the entity-body that would have been sent had the request been a GET

. The meaning would be:

the entity-body that would have been sent if the request had been a GET

, but how come? What has been omitted here? Is it an inversion? What grammatical concept is used here?

Answer 1

This is an example of inversion in conditional clauses.

"If" gets omitted, the auxiliary verb moves to the beginning of the clause.

Had I known that you were coming, I would have cleaned up.

If the verb is a negative ("hadn't"), the "not" particle has to be separated from the auxiliary verb and moved to the side of the main verb.

Had she not helped me I would have been in a lot of trouble. [this example is used in the link above]

An inverted conditional clause sounds more formal and high-flown than a "normal" sequence.