Negational normal form

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Negative normal form is way to represent the formula by keeping the negation symbol only to the literals.

If α and φ are boolean formulas, then α $\wedge$ φ , α $\vee$ φ, $\neg$ α, α $\rightarrow$ φ are all boolean formulas. The boolean formulas are made up of literals which are propositional atoms. A formula can be converted to the NNF by simply moving the negations across braces either using De Morgan's laws or the property that $\neg\neg$ α is α itself.