A run-on sentence consists of two or more grammatical sentences not separated by either a coordinating conjunction (for, and, nor, but, or, yet, so—a.k.a. FANBOYS) or any mark of punctuation this sentence is itself a run-on sentence. To put that definition another way, run-on sentences occur when two or more independent clauses are improperly joined. The most infamous form of the run-on sentence is the comma splice, in which two independent clauses are joined by a comma without a coordinating conjunction.

Let’s look at two examples of run-on sentences. The second one is a comma splice:

• Often, choosing a topic for a paper is the hardest part it’s a lot easier after that.
• Books do not always have the most complete information, it is a good idea then to look for articles in specialized periodicals.

Both of these sentences have two independent clauses. Each clause should be separated from another with a period, a semicolon, a colon, or a comma and a coordinating conjunction:

• Often, choosing a topic for a paper is the hardest part. It’s a lot easier after that.
• OR: Often, choosing a topic for a paper is the hardest part; it’s a lot easier after that.
• Books do not always have the most complete information; it is a good idea then to look for articles in specialized periodicals.
• OR: Books do not always have the most complete information. It is a good idea then to look for articles in specialized periodicals.
• OR: Books do not always have the most complete information, so it is a good idea then to look for articles in specialized periodicals. [This is the “comma and a coordinating conjunction” solution: so is one of the FANBOYS.]

Notice that a run-on sentence can usually be resolved by more than one method.

Common Causes of Run-Ons

As the Fitzgerald example above demonstrates, length is no guarantee of a run-on sentence. The confusion between lengthy and run-on sentences arises perhaps because English speakers know an independent clause when they read it, even if they don’t know its technical name. We pause at the end of independent clauses, recognizing that the grammatical unit of the sentence has come to its end. When another independent clause then follows, without the pause marked by punctuation, it feels as if we have to kick our minds back into gear too quickly, or as if we are being dragged forward unwillingly. That experience makes even a relatively short run-on sentence feel long to us, as in this example: “Jonathan Swift’s last years were unhappy he suffered from Ménière’s disease.”

We often write run-on sentences because we sense that the sentences involved are closely related and dividing them with a period just doesn’t seem right. We may also write them because the parts seem too short to need any division, like in “Swift loved Esther Johnson but he probably did not marry her.” However, “Swift loved Esther Johnson” and “he probably did not marry her” are both independent clauses, so they need to be divided by a comma and a coordinating conjunction—not just a coordinating conjunction by itself; i.e. “Swift loved Esther Johnson, but he probably did not marry her.”

Another common cause of run-on sentences is mistaking adverbial conjunctions (some common examples are however, moreover, therefore, thus, consequently, furthermore, unfortunately) for coordinating conjunctions (the FANBOYS words). For example if we were to write, “Swift loved Esther Johnson, however he probably did not marry her,” we would have produced a comma splice. The correct sentence would read, “Swift loved Esther Johnson; however, he probably did not marry her.” It is unlikely that you would write a five-page college-level essay without using an adverbial conjunction; this pattern, where an adverbial conjunction is preceded by a semi-colon and followed by a comma, is fundamental to complicating a thought, so it should appear in your writing, and you should punctuate it correctly. (Fun fact: adverbial conjunctions are also known as conjunctive adverbs. Regardless of the terminology, punctuate them correctly when you write them.)

Fixing Run-On Sentences

Before you can fix a run-on sentence, you’ll need to identify the problem. You can often trust your own sense that a sentence is dragging; that sense frequently arises when a sentence is run-on rather than lengthy (think back to F. Scott Fitzgerald’s sentence or to the Gary Provost quotation that opens this module; long sentences can be highly effective, but you have to be in control of them). Be alert to adverbial conjunctions and to coordinating conjunctions; you can use your word-processing program’s “find” feature to locate common ones like “however” or “therefore” or “so,” and then check the punctuation. That proofreading might sound onerous, but you won’t have to do it often before you learn to think about those parts of speech while you’re drafting; soon after that, you’ll become competent automatically. And, of course, you don’t need to wait until you’ve written fifteen pages before you start to check; take a look at your first couple of pages slowly and carefully to kickstart the editing process. If trusting your own feel for English and trouble-shooting obvious words still leave you with some dubious sentences, you can isolate them and ask whether the parts are independent clauses. Remember, only independent clauses can stand on their own. This also means they must stand on their own; they can’t run together without correct punctuation. (Yes, this paragraph makes deliberately heavy use of semi-colons to prevent run-ons—we’re glad you noticed!)

Let’s take a look at a few more run-on sentences and their revisions:

1. Some of the hours I’ve earned toward my associate’s degree do not transfer, however, I do have many general education credits the university will accept.
2. Some respondents were highly educated professionals, others were from small villages in underdeveloped countries.
3. The opposite is true of stronger types of stainless steel they tend to be more susceptible to rust.

The first sentence is a comma-splice sentence. The adverbial conjunction however is being treated like a coordinating conjunction. There are two easy fixes to this problem. The first is to turn the comma before however into a period. If this feels like too hard of a stop between ideas, you can change the comma into a semicolon instead.

• Some of the hours I’ve earned toward my associate’s degree do not transfer. However, I do have many general education credits the university will accept.
• Some of the hours I’ve earned toward my associate’s degree do not transfer; however, I do have many general education credits the university will accept.

In the second example, we again have two independent clauses. The two clauses provide contrasting information. Adding a conjunction could help the reader move from one kind of information to another. However, you may want that sharp contrast. Here are two revision options:

• Some respondents were highly educated professionals, while others were from small villages in underdeveloped countries.
• Some respondents were highly educated professionals. Others were from small villages in underdeveloped countries.

The third sentence is a run-on as well. “The opposite is true of stronger types of stainless steel” and “they tend to be more susceptible to rust” are both independent clauses. The two clauses are very closely related because the second clarifies the information provided in the first. The best solution is to insert a colon between the two clauses:

The opposite is true of stronger types of stainless steel: they tend to be more susceptible to rust.

When you are punctuating independent clauses, the difference between a semi-colon and a colon can be subtle, and is not necessarily subject to hard-and-fast rules. Both colons and semi-colons connect independent clauses when the clauses contain closely related material, but we prefer the colon when the second independent clause amplifies, explains, or summarizes the information in the first independent clause. In the example above, the second clause explains what “the opposite” refers to in the first clause.