A run-on sentence joins two or more independent clauses, groups of words that could each be a sentence on their own, without the punctuation or the joining word that would hold them together properly. The reader reaches the end of one thought and is carried straight into the next with no signal that it has begun.
The two kinds
The fused sentence
Two sentences with nothing at all between them:
- I love this café the coffee is excellent.
- The printer is broken nobody has called the technician.
The comma splice
Two sentences with only a comma between them:
- I love this café, the coffee is excellent.
The comma splice is the more common of the two, because the comma feels like a pause and so looks like enough. It has its own article: What Is a Comma Splice?
Long is not the same as run-on
The name suggests a sentence that goes on too long, and many people use it that way. Grammatically, though, length has nothing to do with it. This sentence is long and correct, because its clauses are joined properly:
- When the storm finally passed, we walked down to the harbour, where the boats that had been pulled ashore were being dragged back into the water by fishermen who had waited all night.
And this one is short and still a run-on: It's late I'm leaving.
A very long sentence can still be hard to read, of course, and splitting it often helps. That is a matter of style rather than a grammar error.
How to fix one
First find the clauses: look for each subject with its own verb (I love, the coffee is). Where one clause ends and the next begins, choose a join:
- A full stop: I love this café. The coffee is excellent.
- A semicolon: I love this café; the coffee is excellent.
- A comma and a conjunction (and, but, so, yet, or, for, nor): I love this café, and the coffee is excellent.
- A subordinating word that makes one clause depend on the other: I love this café because the coffee is excellent.
The fourth option often gives the best sentence, because it says how the two ideas are related, which is the information a run-on leaves out.
Where run-ons come from
Run-ons appear most in fast writing: chat messages, notes, first drafts written the way we speak. Speech has no punctuation, only pauses and changes of tone, and those do not survive the trip onto the page. Two habits catch most of them:
- Read the text aloud. Where your voice drops as at the end of a sentence, the page needs more than a comma.
- Watch for pronouns after a comma. A comma followed by it, he, she, they, we or I often starts a new independent clause.
How the spell checker helps
Spotting a run-on means understanding where one clause ends, which is hard for any rule-based check, so do not take a clean result as proof that every sentence is well formed. The Spell Checker takes care of the spelling, grammar and punctuation inside each sentence, and the AI Spell Checker, which reads text in context, is the better tool for sentence structure. A read-aloud pass is the final check; What Is Proofreading? has more techniques.