Phonotactics
Phonotactics (from Ancient Greek phōnḗ "voice, sound" and taktikós "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 /st/ do not occur. Similarly, the clusters /kn/ and /ɡn/ are not permitted at the beginning of a word in Modern English but are in German and Dutch (in which the latter appears as /ɣn/) and were permitted in Old and Middle English. In contrast, in some Slavic languages /l/ and /r/ are used alongside vowels as syllable nuclei.
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. Phonotactics is known to affect second language vocabulary acquisition.