You can break down syllables (sigma) into (1) onset and (2) rhyme/ rime. You can than divide the rhyme into (2.1) nucleus and (2.2) coda. What is the (2.1) nucleus so ? It is the vowel sound or the highest vowel in sonority. The vowel always take the “nucleus position” in a specific word. Every word must have one nucleus. You can only identify the nucleus if you know how to speak properly. The coda and the onset are not obligatory, that is some words don’t have coda or onset. Take for example the word “it”: “i” is the nucleus , because it is the vowel, while “t” is the coda. There is no onset in “it”.