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The Run-on Error (A Common Mistake with Commas)

The Quick Answer
Once you have written a sentence, you must end it with appropriate punctuation such as a full stop (a period), 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.

What Is a 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 full stop / period.

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 full stop / period, question mark, or exclamation mark inside), but it definitely can't end in a comma.)

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.)

  • I always pass on good advice. It is the only thing to do with it. It is never of any use to oneself. (Oscar Wilde)

  • It's not true that I had nothing on. I had the radio on. (Marilyn Monroe)

  • Only one man in a thousand is a leader of men, the other 999 follow women. (Groucho Marx)

  • Be kind to those that meet you as you rise, you may pass them again as you fall.

  • 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)

  • This suspense is terrible. I hope it will last. (Oscar Wilde)

  • Duty is what one expects from others, it is not what one does oneself. (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. (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)

Why Is the Run-on Error So Common?

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

Remember, a sentence is a grammatically complete series of words. Often, it will take several sentences to complete your idea.
Beware

End of Sentence? Well, You Can't Just Put a Comma and Carry On

Once you have formed a sentence (i.e., expressed a complete concept), you must put a full stop / period or another valid ending (like ! or ?).

Do not insert a comma and continue writing. This is a very common mistake. It is known as a run-on error, a run-on comma, or a run-on sentence.
Top Tip

Extending a Sentence with a Semicolon, a Dash, or Three dots

Run-on errors occur because writers feel that their ideas need to be crammed into single sentences. They don't. Occasionally, however, it may be appropriate to use a semicolon, a dash, or three dots (ellipsis) to extend a 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.