AP® English Language and Composition · Big Idea 4 Style: Grammar, Mechanics, and Punctuation (Skills 7.C, 8.C)
8.C Sentence Boundaries: Run-Ons, Splices, and Fragments
A sentence boundary error happens when the punctuation claims the wrong number of complete thoughts. Fixing it means counting how many standing blocks a string of words actually has, then choosing a joint strong enough to hold them.
Before you can name what's wrong with It rained all night the game was cancelled, you feel it: two separate claims are running together with no signal of how they relate. That feeling is your ear catching two independent clauses, each one a complete standing block with its own subject and verb, jammed together with nothing between them. The test is simple: can this chunk of words stand alone as a sentence? If you find two chunks that each pass that test, you have two blocks, and a comma or nothing at all is not a strong enough joint to carry them. You need a full stop, a semicolon, or a comma plus a coordinating conjunction.
Take The power failed, we lit candles. Four repairs all fix the boundary legally: a period (The power failed. We lit candles.), a semicolon (The power failed; we lit candles.), a comma with 'so' (The power failed, so we lit candles), or a comma with 'and'. Each is grammatically sound, but they are not interchangeable in meaning. Only 'so' tells the reader the failure caused the candle-lighting; the others just place the two facts side by side and let the reader infer the link, or state no link at all.
This is why boundary errors matter beyond correctness: the joint you choose is a claim about how two facts relate, so picking the wrong one, or no real joint at all, either hides a relationship the writer meant to show or invents one that isn't there. A fragment flips the same logic. An accidental one, missing a verb or a subject by mistake, reads as a slip. A deliberate one, placed beside full sentences that prove the writer can punctuate correctly, reads as emphasis, because the reader trusts the break was chosen, not stumbled into.
Key terms
- independent clause
- A group of words with a subject and verb that can stand alone as a complete sentence.
- run-on
- Two or more independent clauses joined with no punctuation or conjunction at all.
- comma splice
- Two independent clauses joined by only a comma, which is too weak a joint to carry them.
- fragment
- A group of words punctuated as a sentence but missing a subject, a verb, or a complete thought.
- coordinating conjunction
- A joining word like and, but, or so that can pair with a comma to legally link two independent clauses.
Watch it explained
- How it worksHow do you test a string of words for how many standing blocks it actually contains?
- Worked exampleWalk me through four legal repairs of The power failed, we lit candles and what each one claims.
- Going furtherWhat makes a deliberate fragment work inside an argument when an accidental one does not?
- Ties togetherHow does a sentence-boundary error change the relationship a reader builds between two facts?
Check yourself
End of topic.