Grammar rule

Comma splices and run-ons

Two complete sentences can't be joined by a comma alone.

Updated October 29, 2026

The rule

A complete sentence (an independent clause) has a subject and a verb and can stand alone. Joining two of them with only a comma is a comma splice; joining them with nothing is a run-on.

Fix it with a period, a semicolon, or a comma plus and, but, or, so, for, nor or yet. Words such as however and therefore are not joining words: they need a semicolon or a period before them.

Examples

  • It started to rain, we went inside.It started to rain, so we went inside.
  • The test was long, however, it was fair.The test was long; however, it was fair.

Quick test: Cover each side of the comma. If both could be sentences on their own, the comma needs help.

Try it: find the mistake

The mistake in each sentence is highlighted. Say the fix out loud, then open the answer.

I was hungry, I made a sandwich.

I was hungry, so I made a sandwich.

Two complete sentences can't be joined with only a comma. Add a joining word like so.

My dog is fast he can catch a frisbee in the air.

My dog is fast. He can catch a frisbee in the air.

This is a run-on: two sentences with nothing between them. End the first with a period.

The museum was closed, we went to the park instead.

The museum was closed; we went to the park instead.

Both sides are complete sentences. A semicolon can join them.

The trail was steep, however, we reached the top.

The trail was steep; however, we reached the top.

However is not a joining word. It needs a semicolon (or a period) before it.

Play today's Grammar Fix, free

Where this rule is tested

  • The CLT's Grammar/Writing section and the SAT's Reading and Writing section both test it, usually as "which choice is correct" with four versions of a sentence.
  • In the Daily Grammar Fix, it comes up in sentences for grades 4 to 6, grades 7 to 9 and grades 10 to 12.

Questions people ask

What is the rule for comma splices and run-ons?

Two complete sentences can't be joined by a comma alone. A complete sentence (an independent clause) has a subject and a verb and can stand alone. Joining two of them with only a comma is a comma splice; joining them with nothing is a run-on.

How can I check my comma splices and run-ons quickly?

Cover each side of the comma. If both could be sentences on their own, the comma needs help.

Where can I practice this for free?

In Owl Test Prep's Daily Grammar Fix: one sentence a day for everyone, plus practice sets of five on this rule. It is free, with no account.

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