A grammar is in Chomsky normal form if it has only these kinds of productions:

X ::= Y Z
X ::= t
S ::= ""

where

• X,Y,Z denote non-terminals
• t denotes terminals
• S is the start non-terminal
• if S::=“” appears, then S does not appear on right-hand side of another rule

Observe:

• the empty string can only occur for the start non-terminal
• terminals occur only by themselves on right-hand side
• in parse tree, each non-terminal leads either to terminal or to two other non-terminals