What Is a Run-on Error?

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The Quick Answer
Once you have written a sentence, you must end it with appropriate punctuation such as a period (full stop), an exclamation mark, or a question mark (if it's a question). You cannot put a comma and write another sentence. This is the most common mistake involving commas. It is called a run-on error. For example:
  • I like clowns, they can be scary though.
  • (This should be two sentences. The comma is wrong. This is an example of a run-on error. You might also see the term "comma fault" or "run-on sentence".)
Here are the options for this sentence:
  • I like clowns. They can be scary though.
  • I like clowns; they can be scary though.
  • I like clowns — they can be scary though.
  • I like clowns ... they can be scary though.

Run-On Error

A run-on error is a common writing mistake caused by using inappropriate punctuation at the end of a sentence. Most run-on errors involve writers putting a comma at the end of a sentence and then writing another closely related sentence.

A sentence is a grammatically complete series of words. A sentence must have a subject and a verb, even if one or the other is implied, and it must express a complete concept. A sentence begins with a capital letter and normally ends with a period (full stop).

A sentence can also end in a question mark (?), or an exclamation mark (!). If we're being really pedantic, it could also end in a speech mark (provided there's a period, question mark, or exclamation mark inside), but it definitely can't end in a comma.)

run-on error (writing mistake)

Examples of Run-On Errors

Here are some examples of run-on errors:
  • Pick up a copy of our free brochure, this explains how to contact us and reach our showroom.
  • (This is two sentences. You cannot put a comma after "brochure" and carry on writing.)
  • It's not true that I had nothing on, I had the radio on. (Actress Marilyn Monroe)
  • Only one man in a thousand is a leader of men, the other 999 follow women. (Comedian Groucho Marx)
  • Be kind to those that meet you as you rise, you may pass them again as you fall.
  • This suspense is terrible, I hope it will last. (Playwright Oscar Wilde)
  • Duty is what one expects from others, it is not what one does oneself. (Playwright Oscar Wilde)
  • When will I learn? The answers to life's problems aren't at the bottom of a bottle, they're on TV. (Homer Simpson)

Examples of Sentences Run-On Errors

The trick to avoiding the run-on error is to keep your discipline with what constitutes a sentence. Here are some examples of sentences that too many people would reproduce with run-on errors.
  • I always pass on good advice. It is the only thing to do with it. It is never of any use to oneself. (Irish poet Oscar Wilde)
  • Please don't eat me! I have a wife and kids. Eat them! (Homer Simpson)
  • Please do not shoot the pianist. He is doing his best. (Playwright Oscar Wilde)
  • Drink is the curse of the land. It makes you fight with your neighbor. It makes you shoot at your landlord, and it makes you miss him.
  • (Of course, it is possible to put a comma and a conjunction ("and" in this example) and carry on writing. This is not an error. It is extremely common.)
  • Age is not a particularly interesting subject. Anyone can get old. All you have to do is live long enough. (Groucho Marx)
Would you have used a comma if you were asked to transcribe these quotations? No? Good.

Why Is the Run-on Error So Common?

Writers often feel that a comma is more appropriate than a period (full stop) because their sentences are so closely related. In other words, they sense that a period is too abrupt because they haven't finished expressing their idea.

Why Should I Care about Run-On Errors?

Once you have formed a sentence (i.e., expressed a complete concept), you must put a period or another valid ending (like ! or ?). Do not insert a comma and continue writing. This is a common mistake.

Other Options

Occasionally, it may be appropriate to use a semicolon, a dash, or three dots (ellipsis) to extend a sentence instead of putting a period and writing a second sentence. For example:
  • Duty is what one expects from others; it is not what one does oneself.
  • Please do not shoot the pianist — he is doing his best.
  • The answers to life's problems aren't at the bottom of a bottle ... they're on TV.
Read more about extending a sentence with a semicolon, a dash, or three dots.
Interactive Exercise
Here are three randomly selected questions from a larger exercise, which can be edited, printed to create an exercise worksheet, or sent via email to friends or students.

See Also

Using commas (a summary) Our big commas test What are conjunctions? Commas before conjunctions Commas after a sentence introductions Commas after a transitional phrase Commas after interjections (yes, no, indeed) Commas for parenthesis Commas in lists Commas with a long subject Commas with numbers Commas with quotation (speech) marks Commas with the vocative case Commas with Dear, Hello, and Hi Semicolons to extend a sentence Extend a Sentence (dashes, semicolons and three dots)