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In 1957, Noam Chomsky described five classifications for generative grammars. According to Chomsky:

"�a generative grammar is a system of many hundreds of rules of several different types, organized in accordance with certain fixed principles of ordering and applicability and containing a certain fixed substructure which, along with the general principles of organization, is common to all languages." (Chomsky, 1968, pp. 87-88)There might be grammars that are created by random or stochastic rules, but those are not included in Chomsky's hierarchy. The first classification is the most general category in the hierarchy; the next category is a subset of that one, and so on, down to the most restrictive category.

1. Recursive grammar.

A recursively enumerable
grammar is one that can be instantiated by an algorithm. Another way of saying
this is: "there is a Turing machine that decides it" (Lewis, 1998, p.
195). Given an input that is "valid" for the grammar, the Turing
machine will always halt in a correct "answer" state. When written
using standard notation, both sides of the rewrite rules can have as many
symbols as are needed (example, **A B C D
� M N O P Q** (etc.)). A grammar of this type
could be said to be "incomplete", since the behavior of the Turing
machine is not specified for all inputs. This is the same problem that Bertrand
Russell and Alfred North Whitehead had when constructing their unified formal
logic "Principia Mathematica" (Hofstadter, 1979, pp 17-619).

2. Recursively enumerable grammar.

A subset of #1, a grammar is
recursively enumerable if (and only if) the language *and its inverse* are
both recursive. So, for any input, the Turing machine is guaranteed to halt,
and there are no undecidable inputs that might cause a "halting
problem". Therefore, if **A
� B** is true for a given grammar, **B
� A** is true for
its inverse. This means that given any assumption, we can tell in finite time
whether it is true or false. Recursively enumerable grammars are sometimes
referred to as "type-0" grammars (Manna, 1974, pp. 41-43).

3. Context-sensitive grammars.

Also known as "type-1" grammars, this class is restricted to Turing machines that
can only examine states immediately before and after the current state. So, the
grammar might include a rule to turn a sequence **A B C** into **A X C** (**A B C
� A X B**). Note that there must be the same number of symbols on both the left and right sides
of the rule. Rules that are applied to symbols must consider the
"context" of the symbol. For example, in the previous rule, a "**B**"
can be turned into an "**X**" if (but not necessarily only if) it
has a preceding "**A**" and a following "**C**". This
makes it difficult to modify assumptions since individual symbols cannot be
interpreted on their own. Also, the number of rules needed to fully specify the
grammar might become very large, since there could be many special cases that
need to be considered.

4. Context-free grammars.

Familiarly known as
"CFGs", or "type-2" grammars. Here, there can only be one
nonterminal symbol on the left-hand "assumption" side of the rule.
Luckily, any rule can be rewritten to move all symbols to the right-hand
"conclusion" side as needed (for example, **A � B
� C D** is the same as **A
� B C D**.
Rules can be applied to individual symbols since there is only one given assumption (like the "**A**" in the
previous example). This means "CFGs are popular for natural language
grammars." (Russell, et. al, 1995, p. 656)

5. Regular grammars.

This is the most specific (or
"type-3") grammar and is the smallest subset. To be a regular
grammar, every rule has a single nonterminal symbol on the left-hand side (as
in #4), and only one terminal symbol on the right-hand side (or at most, one
terminal symbol followed by a nonterminal symbol) as in the form **A
� B**. These
grammars are similar to finite state machines, and can be implemented using a
lookup table for each symbol that needs to be translated. However, "they
are poorly suited for programming languages because, for example, they cannot
represent constructs such as balanced opening and closing parenthesis. (Russell, et. al, 1995, p. 656)

Chomsky, Noam, "Language and Mind", Harcourt Brace Jovanovich, San Diego. 1968

Hofstadter, Douglas R., "G�del, Escher, Bach: An Eternal Golden Braid", Vintage Books, New York. 1979

Manna, Zohar, "Mathematical Theory of Computation", McGraw-Hill, New York. 1974

Russell, Stuart J., and Peter Norvig, "Artificial Intelligence: A Modern Approach", Prentice Hall, New Jersey. 1995