A double negative occurs when two or more ways to express negation are used in same sentence. In some languages a double negative resolves to a negative, while in others it resolves to a positive.

A famous linguist once made the further observation that it was unknown for a double positive ever to resolve to a negative. A sceptical voice came from the back of the lecture hall: "Yeah, right". (This joke is often attributed to Prof. Sidney Morgenbesser of Columbia University.)

Different languages have different rules regarding double negative:

Table of contents
1 English
2 Romance
3 Serbian

English

In English, a double negative is a grammatical error if a single negation is meant, for example "I don't want nothing!", meaning "I don't want anything".

Although they are not used in standard English, they are used almost consistantly in African American Vernacular English, and the London Cockney dialect and less frequently, but still commonly, in colloquial English. In the film Mary Poppins, Dick Van Dyke uses a double negative when he says

If you don't want to go nowhere.

(Note: van Dyke's accent in this film is nothing like an authentic English accent)

Other examples of double negatives include:

Don't nobody go to the store.
or
I can't hardly wait.
Double negative also refers to even more than two negatives, like:
And don't nobody buy nothing.
A double negation can be correct, meaning the positive, for example:
It is not unpleasant: logically this is weaker than "It is pleasant", meaning "It is neutral or pleasant", but it is often used as understatement for "It is quite pleasant", a linguistic device called litotes.
Some argue that such constructions are unnecessarily complicated,, so not plain English; and are vaguer in meaning, and therefore not recommendable.

Romance

Romance languages generally express negation by adding a word (ne in French, no in Spanish, não in Portuguese) to the verb and zero or more words elsewhere to indicate what part of the sentence is negated. In French, unlike the others, simple negation usually requires the word pas:
No como.
Je ne mange pas.
Non mangio.
This was originally the same as the word for "step" (Je ne marche pas) and later extended.

The correlative negative words in Spanish and Italian are used only in negative sentences (e.g. ningún - a positive sentence uses algún) whereas some French negative words are the same as positive words (personne means "person" if feminine but "nobody" if masculine). This sometimes leads to confusion if the verb, and therefore the word ne, is omitted.

Serbian

In Serbian, double negative is correct while single negative is grammatical error. The following is a literal translation of a grammatically correct Serbian sentence: "No-one's negligence didn't nowhere never to no-one nohow brought nothing but unhappiness."

See also Double negative elimination