Syllables & Syllable structure
The syllable is a constant feature in every spoken language in the world and most people have an intuitive sense of what a syllable is. Each language has its own rules about what kinds of syllables are allowed, and what kinds aren’t - but the general structure is the same everywhere.
A syllable can have as many as three parts: onset, nucleus, and coda. The onset and the coda are consonants, or consonant clusters, that appear at the beginning and the end of the syllable respectively. The nucleus forms the core of the syllable; it is most often a vowel, or a combination of vowels - but there are exceptions to that.
Source of the picture: spectrum.uni-bielefeld.de
In the word “cat” for example, [c] is the syllable onset, [a] is the nucleus, and [t] the coda. A syllable does not necessarily have to have an onset or a coda - depending on the language - but a nucleus is always present. If a coda is present in a syllable, the nucleus and the coda form a single unit called a rhyme; otherwise the nucleus makes up the rhyme by itself. Looking at “cat” again, [at] forms the rhyme.
Even in English, syllable nuclei are not restricted to vowels. For example, in the monosyllabic word, “hmm”, the syllable nucleus is the nasal consonant [ṃ]. The small dot underneath the character ṃ indicates that the sound represented is a syllabic consonant, which is any consonant that forms a syllable nucleus. Vowels are not marked with the same diacritic because they are always considered to be syllabic.