Generative phonology is a branch of generative linguistics that determines the underlying set of rules governing the pronunciation of words in a person's native language. In general, generative linguistics refers to the theory that all human language is generated from linguistic structures that are hard-wired into the brain at birth. As a person acquires his or her native language, the structures that apply to that particular language are activated. According to the theory of generative phonology then, the person also acquires certain rules about what sounds can be combined in which ways. Phonology is closely related to phonetics, except that phonetics refers to the sounds that are actually produced within a language, rather than the rules that govern the sounds.
The results of generative phonology can be seen in the way one word is derived from another within a language. In English, for instance, the prefix "in-" is often applied to a root word to negate it. In some instances, however, the combination of this prefix and the initial sounds of the root produce a combination of sounds that violates the rules English phonology, so the prefix or root is modified in some way. The word "material," for example, becomes "immaterial" with the prefix "im-" rather than "in-", because the [inma] pronunciation is not allowable. At some point, speakers of the English language modified this word so that it would fit better with the rules of its phonology.
Other instances of generative phonology are not seen in the actual formation of words, but in the way they are pronounced. American English, for instance, contains a phonological rule known as the "flapping rule," which states that a [t] sound becomes a flap [ɾ] before an unstressed vowel. The flap is pronounced more closely to a [d] sound than to a [t] sound. This can be observed when the word "fight" ['fait] becomes "fighter" ['fai dur] and loses the hard [t] sound.
Research in generative phonology involves mapping out phonological rules that are less easily observed than the examples cited above, both in English and other languages. Noam Chomsky's and Morris Halle's 1968 The Sound Pattern of English was the earliest comprehensive work on English phonology, but subsequent research has refined and challenged some of that work's assumptions. Some have used it as a pattern off of which to base phonological research in other languages.