Double negatives are two negative words used in the same sentence. Using two negatives turns the thought or sentence into a positive one. Double negatives are not encouraged in English because they are poor grammar and they can be confusing; but, they are sometimes used in song lyrics and informal speech.
Sometimes a negative can be formed by attaching the prefixes ir-, in-, non- and un-. Here are some examples:
You can create a double negative by using a negative word with a word that acts like a negative. Here are some examples:
To see more double negatives, check out Double Negative Trouble.