The CKY algorithm requires grammars to first be in CNF. Grammars in CNF are restricted to rules of the form A -> B C or A -> w. The right hand-side of each rule must expand either to two non-terminals or to a single terminal.

Situation 1 : mix terminals with non-terminals on the right-hand side

Situation 2 : single non-terminal on the right-hand side (unit productions).

Situation 3 : the length of the right-hand side is greater than 2