When two negatives are used in a single clause, it is called a double negative. Double negative are usually used to stress denial or opposition, as in these examples:
After he had been laid off, Hal realized that he didn’t need none of the luxuries he’d become accustomed to.
(Didn’t and none are negatives. The sentence should be “After he had been laid off, Hal realized that he didn’t need any of the luxuries he’d become accustomed to.”)
That man was not doing nothing but just standing there!
(Not and nothing are negatives. The sentence should be “That man was not doing anything but just standing there!”)
Allison wondered why Jason didn’t call nobody when he became sick.
(Didn’t and nobody are negatives. The sentence should be “Allison wondered why Jason didn’t call anybody when he became sick.”)
Mr. Fowler said he ain’t got none of those apples that you want.
(Ain’t and none are negatives—and you also know that ain’t is considered nonstandard, don’t you? The sentence should be “Mr. Fowler said he did not have any of those apples that you want.”)
You’ll often hear or read double negatives in colloquial speech:
You ain’t heard nothin’ yet!
Ike said he hadn’t seen Betty nowhere.
Properly, these sentences should be “You haven’t heard anything yet!” and “Ike said he hadn’t seen Betty anywhere.”
Grammar Facts
A litotes is a figure of speech that utilizes two negative terms in order to understate a certain fact. For example: “I would not say that he is an unintelligent man.” So while both double negatives and litotes use two negative terms their purposes are different. A double negative creates a meaning the opposite of what it is supposed to mean, while a litotes is used to understate a fact and means exactly what it is supposed to mean. As such a litotes is an acceptable form of a double negative.
Double negative are usually not considered acceptable except in informal language but one exception to the rule comes if you intend a positive or lukewarm meaning. Read this sentence:
I was not unhappy with my recent raise.
The connotation in the double negatives (not and unhappy) tells readers that, while the writer wasn’t unhappy, he or she wasn’t exactly thrilled.
You may also use double negatives if you’re using a phrase or clause for emphasis, as in this example:
“I will not take a bribe, not today, not tomorrow, not any time in my life,” the politician cried.
And then of course there is the classic grammar double negative joke:
A linguistics professor was lecturing to his class one day. “In English,” he said, “a double negative forms a positive. In some languages, such as Russian, a double negative is still a negative. However, there is no language wherein a double positive can form a negative.”
A voice from the back of the room piped up in reply, “Yeah, right.”