Phonotactics

Phonotactics (in Greek phone = voice and tactic = course) is a branch of phonology that deals with restrictions in a language on the permissible combinations of phonemes. Phonotactics defines permissible syllable structure, consonant clusters, and vowel sequences by means of phonotactical constraints.

Phonotactic constraints are language specific. For example, in Japanese, consonant clusters like are not allowed, although they are in English. Similarly, the sounds and  are not permitted at the beginning of a word in Modern English but are in German and Dutch.

Syllables have the following internal segmental structure:
 * Onset (optional)
 * Rime (obligatory, comprises Nucleus and Coda):
 * Nucleus (obligatory)
 * Coda (optional)

Both onset and coda may be empty, forming a vowel-only syllable, or alternatively, the nucleus can be occupied by a syllabic consonant.

English phonotactics
The English syllable (and word) twelfths is divided into the onset, the nucleus , and the coda , and it can thus be described as CCVCCCC (C = consonant, V = vowel). On this basis it is possible to form rules for which representations of phoneme classes may fill the cluster. For instance, English allows at most three consonants in an onset, but among native words under standard accents, phonemes in a three-consonantal onset are limited to the following scheme:


 * + pulmonic + approximant:

This constraint can be observed in the pronunciation of the word blue: originally, the vowel of blue was identical to the vowel of cue, approximately. In most dialects of English, shifted to. Theoretically, this would produce **. The cluster, however, infringes the constraint for three-consonantal onsets in English. Therefore, the pronunciation has been reduced to by elision of the.

Other languages don't share the same constraint: compare Spanish pliegue or French pluie.

Sonority hierarchy
In general, the rules of phonotactics operate around the sonority hierarchy, stipulating that the nucleus has maximal sonority and that sonority decreases as you move away from the nucleus. The voiceless alveolar fricative is lower on the sonority hierarchy than the alveolar lateral approximant, so the combination  is permitted in onsets and  is permitted in codas, but  is not allowed in onsets and  is not allowed in codas. Hence slips and pulse  are possible English words while *lsips and *pusl are not. There are of course exceptions to this rule, but in general it holds for the phonotactics of most languages.

Notes and references
Fonotaktoniezh Phonotaktik Phonotactique Fonotaksi Fonotax