Joining Sentences:
“Run-on” Sentences and Comma Splices

Writers and writing teachers think a lot about joining ideas. In a way, writing is joining ideas. The specific techniques of joining—the punctuation and the joining words (conjunctions)—have therefore evolved to convey the subtle relationships between ideas. Those subtleties can be confusing as you get used to them.

In formal writing, two phrases describe errors—or, I would say, informalities—in putting sentences together: the run-on sentence and the comma splice. I will explain both phrases in detail, but this is the short version: usually, joining two independent clauses requires a conjunction and a comma. Using only the conjunction is called a run-on sentence. Using only the comma is called a comma splice.

Run-on sentences

Many people use the phrase “run-on sentence” to describe a sentence that seems too long, especially one whose length seems to have grown out of the writer’s control. The phrase also has a more precise meaning: it describes two independent clauses joined by a conjunction (often “and” or “but”) without a comma. Examples will help!

This sentence joins two independent clauses:

You used my shampoo without asking, and I drove your car into the pond.

“You used my shampoo without asking” could stand alone as a sentence, and so could “I drove your car into the pond.” Each of them is therefore an independent clause. (An independent clause has its own subject and verb and could stand on its own as a sentence.)

A run-on sentence would join the two independent clauses with only the conjunction (in this case, “and”), like this:

You used my shampoo without asking and I drove your car into the pond.

Note that the only difference between this sentence and the original example is that the comma has disappeared. It becomes a run-on sentence because it lacks the comma, not because it is especially long. You can think of a run-on sentence as “running on” too quickly through the place where a comma should go.

Comma splices

A comma splice joins two independent clauses with only a comma. (In other words, the comma functions to splice the two independent clauses.) A comma splice looks like this:

I am mad at you, your turtle ran over my foot.

In formal writing, the two independent clauses require something more than a comma to join them. For example, you could simply use a period to make each independent clause its own sentence:

I am mad at you. Your turtle ran over my foot.

That works well. You may wish, however, to clarify that the anger has arisen because of the turtle incident. A semicolon expresses that logical connection:

I am mad at you; your turtle ran over my foot.

You can also make the logic explicit by using a conjunction such as “and” or “but.” (You can remember common conjunctions with the acronym FANBOY: for, and, nor, but, or, yet.) In this case, reversing the order of the independent clauses might help:

Your turtle ran over my foot, and I am mad at you.

Alternatively, you can use words like “therefore” or “however” (conjunctive adverbs). These words operate similarly to the FANBOYs, except they require periods or semicolons rather than commas:

Your turtle ran over my foot. However, I am not mad at you.

Your turtle ran over my foot; however, I am not mad at you.

All of these cases have the same underlying message: you need more than a comma to connect two independent clauses. To get to that something more, you can replace the comma with a semicolon or a period, or you can add another word—a FANBOY or a conjunctive adverb such as “therefore” or “however.”

If you have gotten this far, you have absorbed a lot of information about the length and construction of sentences. You're ready to test your text.