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Chomsky normal form*

A simplified form of grammars where all the production rules are of the form A → BC or A → a or S → ε, where A, B and C are nonterminals (with neither B nor C being the start symbol), a is a terminal, ε is the empty string, and S is the Start symbol. The third rule is only applicable if the empty string is in the language. Any Context-free grammar can be converted to this form, and any grammar in this form is context free.

[Wikipedia]