Independent clause

An independent clause is a group of words with its own subject and finite verb that could stand alone as a sentence — 'the lock turned' — and it is the unit that compound, complex and compound-complex sentences are counted in.

The lock turned. Subject, verb, complete thought — that is an independent clause, and it is still one when it is buried inside a longer sentence. In Although the key was bent, the lock turned, the bold part remains independent; the clause in front of it is not.

Counting these is how the sentence types are defined. One independent clause is a simple sentence. Two joined together make a compound sentence. One plus a dependent clause makes a complex sentence.

The practical payoff is the punctuation. Two independent clauses cannot be joined by a comma alone. The lock turned, the door stayed shut is a comma splice. Four legitimate repairs:

• Full stop: The lock turned. The door stayed shut.
• Comma plus a conjunction: The lock turned, but the door stayed shut.
• Semicolon: The lock turned; the door stayed shut.
• Subordinate one of them: Though the lock turned, the door stayed shut.

Each says something slightly different about how tightly the two facts belong together, which is why "fix the comma splice" is a rhythm decision as much as a correctness one. The splice is occasionally deliberate — in rushed first-person narration it can read as breathlessness — but a reader has no way to tell your intent from an error, so earn it.