A double negative occurs when two forms of negation are used in the same sentence. Multiple negation is the more general term referring to the occurrence of more than one negative in a clause.
In most logics and some languages, double negatives cancel one another and produce an affirmative sense; in other languages, doubled negatives intensify the negation. Languages where multiple negatives intensify each other are said to have negative concord. Portuguese, French, Persian, and Spanish are examples of negative-concord languages, while Latin and German do not have negative concord. Standard English lacks negative concord, but it was normal in Old English and Middle English, and some modern dialects do have it (e.g. African American Vernacular English and Cockney), although its usage in English is often stigmatized.
Languages without negative concord typically have negative polarity items that are used in place of additional negatives when another negating word already occurs. Examples are "ever", "anything" and "anyone" in the sentence "I haven't ever owed anything to anyone" (cf. "I haven't never owed nothing to no one" in negative-concord dialects of English, and "Nunca devi nada a ninguém" in Portuguese, lit. "Never have I owed nothing to no one"). Note that negative polarity can be triggered not only by direct negatives such as "not" or "never", but by words such as "doubt" or "hardly" ("I doubt he has ever owed anything to anyone" or "He has hardly ever owed anything to anyone").
Stylistically, in English, double negatives can sometimes be used for understated affirmation (e.g. "I'm not feeling bad" vs. "I'm feeling good"). The rhetorical term for this is litotes.
Read more about Double Negative: English, Germanic Languages, Romance Languages, Welsh, Greek, Slavic Languages, Baltic Languages, Uralic Languages, Japanese, Chinese, History of Languages
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