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.
Ready for the Test?
Here is a confirmatory test for this lesson.

This test can also be:
  • Edited (i.e., you can delete questions and play with the order of the questions).
  • Printed to create a handout.
  • Sent electronically 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)