A double negative is usually produced by combining the negative form of a verb (e.g., cannot, did not, have not) with a negative pronoun (e.g.,
nothing, nobody), a negative adverb (e.g., never, hardly) or a negative conjunction (e.g., neither/nor).
Examples of Double Negatives
Here are some examples of double negatives:
I didn't see nothing.
I did not have neither her address nor her phone number.
It wasn't uninteresting.
She is not unattractive.
A double negative gives the sentence a positive sense. For example:
"He didn't see nothing." = "He saw something."
"She claims she has not seen neither Paul nor John." = "She claims she has seen either Paul or John."
Often, the positive sense is not what the speaker is trying to say, but a double negative is not always an error. Look at this example:
"She is not unattractive." = "She is attractive."
(Of course, not unattractive could also mean average looking. It depends on context.)
When used to mean attractive, the double negative not unattractive carries a connotation of the speaker being factual as opposed to complimentary.
A Double Negative Is Usually an Error
A double negative is usually an error because it portrays a positive sense when a negative one is intended. In reality, readers nearly always understand the intended meaning, but a writer's credibility is always damaged when a double-negative error is made.
"The secret to being a likeable grammarian is knowing when to shut up."
What about a Triple Negative?
You do not see triple negatives often, but here is a witty one:
I cannot say that I do not disagree with you.
(This quote by Groucho Marx is a triple negative. If
you follow it through logically, you'll find it means I disagree with you. Genius!)