Phonotactics is the particular combination of letter sounds that are allowable within a given language. Each language, or even each dialect of a language, has its own set of rules that speakers stay within. Phonotactics is a branch of phonology, the study of the sound structures of languages, but also has applications in phonetics, the actual production of sound, in synthesized speech and language identification.
A language's phonotactics is comprised of the sounds and placement of sounds that will be found in its words. English and Swahili, for instance, are governed by very different sound rules. In English, a word can end with the sound "ng," as in "sing," but it cannot begin with that sound. Swahili and other Bantu languages, however, can begin words with "ng." Swahili also frequently uses the letter combination "mz," such as in mzee, which means "old man." This combination of sounds is very rarely found in English and never at the beginnings of words.
Phonotactics also affects the structure and emphasis of syllables in a language. Nearly every French word, for instance, has an emphasis on the final syllable. In Greek, the emphasis depends on the length of the final vowel in the word, among other factors. When speaking his or her native language, a person is often able to put the emphasis on the correct syllable intuitively, even if reading an unfamiliar word.
Mastering the phonotactics of a new language is a large part of learning to speak clearly and accurately in that language. For instance, when presented with the word mzee in Swahili, a non-native speaker might be tempted to insert a vowel between the two consonants and pronounce it "muzee." A speaker more adept in Swahili phonotactics is less likely to make this error.
Linguists use the study of phonotactics in a variety of practical applications. A computer that has been programmed to understand the phonological structures of a given language can apply text-to-speech technology to provide reasonably intelligible speech output. It will be able to put stresses on the correct syllable of a sentence the majority of the time, for example, even when the input contains unfamiliar words.
Another use of phonotactics is in language identification. Most people would be able to differentiate between Chinese and French if they heard a sample of each, even if they do not actually speak either language. One goal of research in this area is to teach a computer to do the same: to recognize a language-based syllable structure and letter combinations. This technology would potentially have uses in human services, government and other areas.