Compound sentence

A compound sentence joins two or more independent clauses of equal weight, linked by a coordinating conjunction, a semicolon, or a colon — 'the lock turned and the door swung in'.

The lock turned, and the door swung in. Two clauses that could each be a sentence, joined as peers. The coordinating conjunctions are and, but, or, nor, for, so, yet; a semicolon does the same job with more pause and no stated relationship.

Coordination makes a claim about the two halves: they matter the same amount. That is the right claim surprisingly often — He rang the bell, and nobody came gives both facts their due — and the wrong one whenever a sentence is really about one thing with a condition attached. When every sentence on a page is compound, the prose reads as a list of equally important events, which is another way of saying nothing on the page is important. The fix is usually a complex sentence: demote one clause and the other rises.

Two variants worth knowing. Drop the conjunctions entirely and you get asyndeton — The lock turned, the door swung in, the hall was empty. Repeat them past the point of need and you get polysyndeton — and the lock turned and the door swung in and the hall was empty. Same clauses, three different speeds.