Phonotactics

Phonotactics 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". The study of
phonotactics is a central topic in phonology.
Phonotactics patterns in which the phonemes of a language may
combine to form sequences.