SAT® · Reading and Writing: Boundaries — Between Sentences
Fused Sentences and Run-Ons
A run-on happens when two independent clauses get jammed together with no joint between them. You fix it by finding where the second clause starts and dropping in a period, semicolon, or comma plus conjunction.
Read 'The volcano erupted the village evacuated' out loud and your ear stumbles, not because any word is wrong, but because two complete sentences are lying side by side with nothing holding the seam together. 'The volcano erupted' is a full idea. 'The village evacuated' is a full idea. Mash them together with zero punctuation and the reader has no signal that one thought ended and another began, so the whole thing reads as one long pileup of words.
To catch this, sweep the sentence left to right looking for a second subject-verb pair that could stand on its own. The moment you find one sitting right next to the first clause with no mark between them, you have found the fusion point, the exact spot where the sentence collapsed. Take 'Glaciers carve valleys they leave moraines behind as they retreat.' Scan it: 'Glaciers carve valleys' is complete. Right after it sits 'they leave moraines,' also complete, with nothing but a space separating them. That gap is the break point, and it needs a period, a semicolon, or a comma plus a coordinating conjunction like 'and.'
The fix is never about shortening the sentence. 'Rain fell crops failed' is only four words and still a run-on, because length has nothing to do with it; missing joints do. This topic sits right next to comma splices and fragments in the same family of boundary errors: splices add a weak comma where a strong joint belongs, fragments cut a sentence short before it finishes, and run-ons skip the joint entirely. All three ask the same question: does this chunk of words stand as one complete sentence or does it need to be split or joined differently?
Key terms
- independent clause
- A group of words with a subject and verb that expresses a complete thought and could stand alone as a sentence.
- run-on sentence
- Two or more independent clauses joined with no punctuation or connecting word between them.
- fusion point
- The exact spot in a run-on where one complete clause ends and the next begins with no joint.
- coordinating conjunction
- A connecting word like and, but, or so that can join two independent clauses when paired with a comma.
- comma splice
- A related error where two independent clauses are joined by a comma alone, with no conjunction.
Watch it explained
- How it worksHow do two independent clauses jammed together with no joint let the whole structure collapse?
- How it worksHow do you sweep a long sentence left to right hunting for the second subject that starts a new clause?
- Worked exampleWalk me through locating the fusion point in 'Glaciers carve valleys they leave moraines behind as they retreat'.
- Common mistakeWhy isn't a run-on just a sentence that got too long, given that a four-word sentence can be one?
Check yourself
End of topic.