Phonotactics (from Ancient Greek "voice, sound" and "having to do with arranging") 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 phonotactic constraints. Phonotactic constraints are highly language-specific. For example, in Japanese, consonant clusters like do not occur. Similarly, the clusters and are not permitted at the beginning of a word in Modern English but are in German and Dutch (in which the latter appears as ) and were permitted in Old and Middle English. In contrast, in some Slavic languages and are used alongside vowels as syllable nuclei. Syllables have the following internal segmental structure: Onset (optional) Rhyme (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. Phonotactics is known to affect second language vocabulary acquisition. English phonotactics The English syllable (and word) twelfths is divided into the onset , the nucleus and the coda ; thus, it can 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 (and excluding a few obscure loanwords such as sphragistics), phonemes in a three-consonantal onset are limited to the following scheme: + stop + approximant: + + stream + + (not in most accents of American English) stew + + sputum sprawl splat + + skew scream sclerosis squirrel 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 in what is known as yod-dropping. Not all languages have this constraint; compare Spanish or French . Constraints on English phonotactics include: All syllables have a nucleus No geminate consonants No onset No in the syllable coda (except in Hiberno-English) No affricates or in complex onsets The first consonant in a complex onset must be an obstruent (e.g. stop;