The syllable is a constant feature in every spoken language in the world. 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 has 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 the core of the syllable; it is most often a vowel, or a combination of vowels - but there are many exceptions to that. If you examine enough languages you can find almost every kind phone used as a syllable nucleus. In the word “far”, [f] is the syllable onset, [a] is the nucleus, and [r] the coda. 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 “far” again, [ar] forms the rhyme. A syllable does not necessarily have to have an onset or a coda - depending on the language - but a nucleus is always present.
Even in English, syllable nuclei are not restricted to vowels. For example, in the monosyllabic word, “hmm”, the syllable nucleus is [ṃ], which is a consonant but is more specifically a nasal. Another nasal, [n], can be seen as a syllable nucleus in the word “isn’t”, which is transcribed into the International Phonetic Alphabet (IPA) as [ɪzzṇt]. In this case there are two syllables, and [ṇ] forms the nucleus of the second syllable.
The small dot underneath the characters ṇ and 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. Usually syllabicity is marked in IPA with a vertical stroke under the character instead of a dot; but in this case a dot was as close as I could get.
Anyone following closely may notice that it is difficult to decide exactly how to divide “isn’t” into syllables. It seems like it should be either [‘ɪz.ṇt], or [‘ɪ.zṇt], where the dot (.) represents a syllable boundary and the apostrophe (‘) represents the beginning of a stressed syllable; but it is tricky to figure out whether the [z] is the coda of the first syllable or the onset of the second syllable. That is because in the [z] in “isn’t” demonstrates ambisyllabicity, a common feature in English that I will write about in a future article. The short explanation is that [z] is both an onset and a coda, which is why I transcribed the word with two [z]’s. Anyway, in both analyses [ṇ] forms the nucleus of the second syllable, so we don’t need to worry about the placement of [z] for now.
There are also arguably cases where a liquid forms the nucleus of a syllable in English. The liquids in English are [l] and [r]. Consider the word “sir” and the second syllable of “apple”. If it is the case that the liquids in these syllables are the nuclei, then the words would be transcribed as [sṛ] and [æppḷ] respectively. However, not everybody agrees with that interpretation, and so these words are often analyzed as [sər] and [æppəl]. That would make the syllable nuclei [ə], which is a vowel. And by the way, the [p] in “apple” is also ambisyllabic.
The reason vowels are so likely to form syllable nuclei is that they are the most sonorous sounds available in spoken language. It is a general rule that syllable nuclei are formed by sonorous phones. Liquids and nasals are right behind vowels in that they are more sonorous than any other type of consonant. But it is possible to find syllable nuclei in other languages that are considerably less sonorant than any of the above. For example, there is at least one Berber language that contains syllables like [tḳt], where the nucleus of the syllable is [ḳ].
The onset and coda are always made up of consonants. Many languages have strict rules about how many consonants can appear, and what sort of order they appear in. English is relatively lax in this respect, so we can see syllables with several consonants clustered together in both positions. For example, “scrumptious”, transcribed as [‘skrʌmp.ʃəs], has three distinct consonant phones in the onset and two in the coda of the first syllable. But notice that you would never see a word in English like [‘rksʌpm.ʃəs], which would probably be spelled “rksupmtious”. Try pronouncing that and see what happens - and remember that it’s cheating to make the [r] or the [m] into separate syllables!
Consonants in syllable onsets and codas are also governed by sonority. There is a rule, called the sonority sequencing generalization, that says that the sonority of a syllable peaks at the nucleus and decreases toward either boundary. So the sonority of consonants in the onset is supposed to increase going forward, and the sonority in the coda is supposed to fall of. This is a generalization, so there are exceptions - but if you deviate too much from the rule the result becomes difficult to pronounce. In the made-up word above I reversed the order of the consonants in the onset and coda of the first syllable, thus making the sonority sequence “wrong”.
Every sound in a language has a place somewhere in the sonority hierarchy. And every language has its own sonority hierarchy ordering. So for example [star] is easy for English speakers to pronounce, but you don’t see a word like [tsar]: [t] is more sonorous than [s] in English. But in Russian it is not uncommon to see a word like [tsar].
Restrictions on what is allowed in a syllable vary from language to language. Not all languages allow as many consonants to be clustered in a syllable onset as English does, while some allow more. Some languages require every syllable to have an onset, while others allow naked nuclei. Syllable codas are especially restricted. For example, in Mandarin Chinese the only syllable codas that are allowed are nasals. There are no languages that forbid onsets, but many languages don’t allow syllable codas to appear at all.