Phonology is the study of sound systems, or the sounds of language in context and the rules languages apply to them. It is related to phonetics.
Phonotactics[]
Phonotactics is the field of syllable formation, and investigates what syllables are or are not permissible in a language.
All human languages allow at least the syllable structure CV (Consonant-Vowel). Most also allow word-initial vowels.