A grammar G is 4-tuple or quadruple G=(V,T,P,S) where
V is set of variables or non-terminals.
T is set of terminals.
P is set of productions.
S is the start symbol.
Each production is of the form α -> β where α is a non empty string of terminals and/or non-terminals and β is string of terminals and/or non-terminals including the null string. This grammar is also called as phase-structure grammar.