Complex sentence

A complex sentence contains one independent clause and at least one dependent clause — 'although the key was bent, the lock turned' — so the two halves are deliberately unequal in weight.

Although the key was bent, the lock turned. One clause stands on its own and one leans on it, and the leaning is the whole point: a complex sentence says this part matters more than that part.

Compare the same two facts as a compound sentence: The key was bent, but the lock turned. Now they are equals, and the reader has to decide for themselves which one the scene is about. The complex version has already decided.

Order is a second lever. Put the dependent clause first and the reader waits for the payload, which builds a small pressure — the basic move of the periodic sentence. Put it last and the sentence lands its point and then qualifies it: The lock turned, although the key was bent.

Complex sentences are what make a page of prose feel like it was thought about. A writer who only ever writes simple and compound sentences is handing the reader an unsorted pile; a writer who never writes anything but complex sentences is sorting out loud, at length, and the page slows to a crawl. The mix is the style.