Grammar

A  grammar  is a set of  rules  that defines a legal class of strings in a formal language. A grammar
  is a four-tuples (N, Σ,P,S)
grammar G is written as follows:
G = <N, Σ,P,S>

where:
N - a finite set of non terminal symbols  
Σ - a finite set of terminal symbols
P - a finite set of production rules of the form y->x,where y and x are 
     in (NUΣ) and y contains at least one element in  N
S - is a designated goal system in N

 N and Σ are disjoint.