A constituent structure grammar (context-free grammar, Chomsky Type II grammar) defines tree-hierarchies of parts of a string of symbols. Such strings are often used to represent sentences, and the symbols are often used to represent words. In this way, a context-free grammar can be regarded as a theory which describes the part-whole relations involved in sentence parsing.
A rule in a constituent structure grammar has one non-terminal symbol (variable symbol) on the left hand side (LHS), and one or more terminal symbols (constant symbols) or non-terminal symbols on the right hand side (RHS).
The more restricted type used here is called a Chomsky Normal Form (CNF) grammar; it has only one or two non-terminal symbols on the right hand side, or only one constant symbol on the right hand side. In addition, it has rules with only one constant symbol on the right hand side.
In addition to listing the immediate constituents of a unit, the order (in European writing, from left to right, in speech from earlier to later) is also modelled by the order of the RHS symbols.
In the illustrative grammar given below in Prolog, the following kinds of information are given: