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.
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 among native words under standard accents, phonemes in a three-consonantal onset are limited to the following scheme:
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].