Comma Splices and Run-On Sentences

July 22, 2025

Posted by STLCC in Writing Resources 

Comma splices and run-ons are major sentence errors that interfere with efficient transmission of thought from writer to reader.

• A comma splice occurs when only a comma separates two independent sentences.
• A run-on (or fused sentence) occurs when no punctuation at all separates independent sentences.

Explanation

Let’s start with two simple sentences:

Jasmine is a college student. She studies microbiology.

Both of these simple statements pass the “independent sentence” test: Each one contains a subject and a verb; each one also expresses a complete thought.

Where Things Go Wrong

Although this seems pretty straightforward, in the heat of unloading ideas from brain to essay, writers often become lax in clearly indicating where one complete idea ends and where the next begins, as well as how they perceive the relationship between those ideas, often leading to sentence-level errors like these:

Jasmine is a college student, she studies microbiology. (comma splice)
Jasmine is a college student she studies microbiology. (run-on sentence)

As you can see in these two sentences, comma splices and run-on sentences are similar mistakes. In both cases, the punctuation (or lack of it) does not indicate complete, independent sentences.

A comma splice or run-on sentence can also occur when you use transition words:

The U.S. Postal Service is usually very reliable, however, sometimes a letter is not delivered for weeks, months, or even years.

Again, there are two independent sentences in the example above. Transition words such as however, therefore, nevertheless, consequently, and then (officially referred to as adverbial conjunctions) do hint at the relationship between ideas; however, they cannot be used alone to connect two sentences.

Corrected versions of the previous sentences could be:

The U.S. Postal Service is usually very reliable; however, sometimes a letter is not delivered for weeks, months, or even years.
The U.S. Postal Service is usually very reliable. However, sometimes a letter is not delivered for weeks, months, or even years.

Avoiding and Correcting Comma Splices and Run-Ons

There are four basic ways to fix comma splices and run-on sentences:

1. Separate the sentences with a period and a capital letter.

   Sentence. Sentence.

   Run-on: Saturday afternoon in Kirkwood is always a busy time everyone seems to be out running errands.

   Corrected: Saturday afternoon in Kirkwood is always a busy time. Everyone seems to be out running errands.

2. Join the sentences with a comma and a coordinating conjunction.

   Use one of the coordinating conjunctions: for, and, nor, but, or, yet, so.

   Sentence, and sentence.

   Incorrect: Communication is important in every relationship, it is especially important to a newly married couple.

   Corrected: Communication is important in every relationship, but it is especially important to a newly married couple.

3. Join the sentences with a semicolon.

   A semicolon connects two complete sentences that are closely related. You can add a transitional word or phrase after the semicolon.

   Sentence; sentence.
   Sentence; transition, sentence.

   Incorrect: Suddenly, complete silence descended on the boys' room then the mother knew she'd better check on them.

   Corrected: Suddenly, complete silence descended on the boys' room; the mother knew she'd better check on them.

   Corrected with transition: Suddenly, complete silence descended on the boys' room; then the mother knew she'd better check on them.

4. Use a subordinating conjunction to combine the ideas into a complex sentence.

   A subordinating conjunction connects a less important idea to the main idea. The dependent clause can come before or after the main clause.

   Subordinating conjunction + subject/verb, sentence.
   Sentence subordinating conjunction + subject/verb.

   Incorrect: The surgeon was in complete control of the situation at no time was the patient in danger.

   Corrected: Because the surgeon was in complete control of the situation, at no time was the patient in danger.

   Corrected: At no time was the patient in danger because the surgeon was in complete control of the situation.