Context Free Grammar
A context Free Grammar (CFG) is a 4tuple such that
G = (V , T , P , S)
where
 V = Finite nonempty set of variables / nonterminal symbols
 T = Finite set of terminal symbols
 P = Finite nonempty set of production rules of the form A → α where A ∈ V and α ∈ (V ∪ T)*
 S = Start symbol
Why Context Free Grammar Is Called So?
Context Free Grammar provides no mechanism to restrict the usage of the production rule A → α within some specific context unlike other types of grammars. That is why it is called as “Context Free” Grammar. 
Example01:
Consider a grammar G = (V , T , P , S) where
 V = { S }
 T = { a , b }
 P = { S → aSbS , S → bSaS , S → ∈ }
 S = { S }
 This grammar is an example of a context free grammar.
 It generates the strings having equal number of a’s and b’s.
Example02:
Consider a grammar G = (V , T , P , S) where
 V = { S }
 T = { ( , ) }
 P = { S → SS , S → (S) , S → ∈ }
 S = { S }
 This grammar is an example of a context free grammar.
 It generates the strings of balanced parenthesis.
Applications
Context Free Grammar (CFG) is of great practical importance. It is used for following purposes
 For defining programming languages
 For parsing the program by constructing syntax tree
 For translation of programming languages
 For describing arithmetic expressions
 For construction of compilers
Context Free Language
The language generated using Context Free Grammar is called as Context Free Language. 
Properties
 The context free languages are closed under union.
 The context free languages are closed under concatenation.
 The context free languages are closed under kleen closure.
 The context free languages are not closed under intersection and complement.
 The family of regular language is a proper subset of the family of context free language.
 Each Context Free Language is accepted by a Pushdown automaton.
RememberIf L1 and L2 are two context free languages, then

Ambiguity in Context Free Grammar
A grammar is said to be ambiguous if for a given string generated by the grammar, there exists
 more than one leftmost derivation
 or more than one rightmost derivation
 or more than one parse tree (or derivation tree).
Read More Grammar Ambiguity
To gain better understanding about Context Free Grammar,
Next Article Chomsky Normal Form
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