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 /st/ are not allowed, although they are in English. Similarly, the sounds /kn/ and /ɡn/ 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:
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 /twɛlfθs/ is divided into the onset /tw/, the nucleus /ɛ/, and the coda /lfθs/, 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 phonemes in a three-consonantal onset are strictly limited to a subset of the following scheme:
- /s/ + voiceless plosive (/p t k/) + approximant (/ɹ j w/)
- /s/ + voiceless plosive (/p k/) + approximant (/l ɹ j w/)
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 [iw]. In most dialects of English, [iw] shifted to [juː]. Theoretically, this would produce **[bljuː]. The cluster [blj], however, infringes the constraint for three-consonantal onsets in English. Therefore, the pronunciation has been reduced to [bluː] by elision of the [j].
Note that those English dialects that preserve the pronunciations [kiw] for cue have also preserved the pronunciation [bliw] for blue. Other languages don't share the same constraint: compare Spanish pliegue [ˈpljeɣe] or French pluie [plɥi].
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 [s] is lower on the sonority hierarchy than the alveolar lateral approximant [l], so the combination /sl/ is permitted in onsets and /ls/ is permitted in codas, but /ls/ is not allowed in onsets and /sl/ is not allowed in codas. Hence slips /slɪps/ and pulse /pʌls/ 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.