Generative Grammar - Context-free Grammars

Context-free Grammars

Generative grammars can be described and compared with the aid of the Chomsky hierarchy proposed by Noam Chomsky in the 1950s. This sets out a series of types of formal grammars with increasing expressive power. Among the simplest types are the regular grammars (type 3); Chomsky claims that regular grammars are not adequate as models for human language, because all human languages allow the center-embedding of strings within strings.

At a higher level of complexity are the context-free grammars (type 2). The derivation of a sentence by a grammar can be depicted as a derivation tree. Linguists working in generative grammar often view such derivation trees as a primary object of study. According to this view, a sentence is not merely a string of words, but rather a tree with subordinate and superordinate branches connected at nodes.

Essentially, the tree model works something like this example, in which S is a sentence, D is a determiner, N a noun, V a verb, NP a noun phrase and VP a verb phrase:

The resulting sentence could be The dog ate the bone. Such a tree diagram is also called a phrase marker. They can be represented more conveniently in text form, (though the result is less easy to read); in this format the above sentence would be rendered as:
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Chomsky has argued that phrase structure grammars are also inadequate for describing natural languages, and has formulated the more complex system of transformational grammar.