See also:

• Computers: Computer Science: Theoretical: Automata Theory: Pushdown Automata  (5)

• Context-free Grammar - A Wikipedia article that defines context free grammars and uses them to generate context free languages.
• Formal Languages and Compilers - Course notes onthe definition of formal languages and the use of context free languages in compiler construction.
• Formal Grammars and Languages - A survey article on formal systems that define families of formal languages arising in many computer science applications with primary focus on context-free languages.
• Push-Down Automata and Context-Free Grammars - Lecture notes defining context free grammars and closure and decidability properties of context free languages. There is a short section showing that natural languages are not context free.