Essentially, we can divide grammars into two types. Prescriptive grammars prescribe or dictate how we should use the language. These kinds of grammar were traditionally used in first language education, and contain judgmental views on what constitutes 'good' and 'bad' English, for example. Prescriptive approaches often assume that some dialects are superior to others, so that in the example below, 'I'm not' would be seen as more 'standard':

I ain't.
I'm not.
I amn't.

To a large extent, traditional prescriptive grammars were based on the study of Latin. For example, the rule I myself learned as a child that one should never split infinitives, as in 'to boldly go', derives from the fact that in Latin all infinitives are single words (as they are in modern Spanish, French, etc), and therefore cannot be split. Similarly, the rule that one should never end a sentence with a preposition also comes from the fact that Latin sentences never end in prepositions. The difficulties involved in trying to impose rules on a language are well known:

The grammarian of our own language considered that his function was to settle usage and control development - but from the point of view of the scientific grammarian, who looks on language as a thing that lives and grows and changes, he might as well set himself to control the tides or the winds. (Kittson, 1918: 23)

For the purposes of second language (L2) acquisition, grammars are largely prescriptive in that they try to teach learners how to use grammar accurately, usually in accordance with native speaker models.

Descriptive grammars describe how we actually speak, ie how we use communicatively the rules stored in our mental grammars. A descriptive grammar will attempt to uncover rules of grammar from examinations of real-world language use, rather than lay down these rules according to socially determined norms. For a descriptive grammarian, the following concepts are key:

• grammar 'rules' do not tell us how we should use the language, but describe a pattern observed in our use of language;
• no one language or dialect is superior to any other;
• the grammars of all languages are equally complex and capable of producing any thought and an infinite number of sentences.

The best examples of descriptive grammars are those based on analyses of large-scale computer databases of language, known as corpora. The COBUILD dictionaries for learners of English are based on a corpus of over 300 million words, as is Hunston's (2000) work on pattern grammar.

From our perspective as language teachers, what we need is a pedagogic or teaching grammar. As we have noted, this will be largely prescriptive, as it will need to serve as a guide to learners on how to use the language accurately. At the same time, it will be based on a description of the way certain designated speakers of the target language use the language.