A double negative happens when two negative words show up in the same clause. In standard English, two negatives cancel each other out and produce a positive meaning — which is usually the opposite of what the writer meant to say.
Mathematically and logically, two negatives cancel: "not un-happy" means "happy." Standard English grammar applies that same logic to sentences, which is why "I don't know nothing" is read as "I do know something" rather than as extra emphasis on not knowing.
"Hardly," "barely," and "scarcely" already function as negatives in English, so pairing them with "not" or "can't" creates the same cancel-out problem.
This construction is technically a double negative too, but it's more often used deliberately for a specific effect — softening a claim ("not unreasonable" implies "reasonable, but I'm hedging") rather than stating it directly. It's grammatically fine, but worth using on purpose rather than by accident, since readers can genuinely misread which way you meant it.
Some dialects and informal registers use double negatives for emphasis rather than cancellation — "I ain't got no money" is understood as strongly negative in casual speech, not as a roundabout way of saying you have money. That's a dialect feature, not a mistake, but it reads as nonstandard in formal or professional writing, where the cancel-out reading is the expected one.
Double negatives are easy to miss because the sentence "sounds" negative when you read it back, even though the grammar says otherwise. Proofread Bot flags double negatives and clarifies which meaning your sentence is actually expressing.
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