A run-on, also called a fused sentence, is the opposite of a fragment: two independent clauses run together with no punctuation and no joining word between them. You learned how to join clauses properly in Simple and Compound Sentences.
Key idea
A run-on has two independent clauses with nothing joining them. It is about how clauses are joined, not about length: a short sentence can be a run-on, and a long sentence can be correct.
Why it works
Without a signal, the reader can’t tell where one thought ends: “Mia called her brother answered” reads at first as if Mia called her brother. Punctuation shows the border.
Worked example: Find the border
“The store was closed we went home.”
- Look for a second subject and verb: we went.
- The first clause ends just before it, after closed.
- Nothing marks the border, so it is a run-on. The simplest fix is a period.
Answer: The store was closed. We went home.
Worked example: Long but correct
“My grandmother, who grew up on a farm in Iowa, still wakes up before sunrise and makes coffee for everyone in the house.”
- Find the subjects and verbs: grandmother wakes … and makes; who grew up is inside a dependent clause.
- Only one independent clause.
- Long, but not a run-on.
Answer: Correct as written.
Common mistake
Thinking a run-on means a long sentence, and chopping good long sentences into short ones. Test: count the independent clauses, then check what joins them. Ways to fix them come in Five Ways to Fix a Run-On.