Let’s look at how this works in some English words. When we say a word is “monosyllabic” that just means that it has one syllable. We’ll start with a nice simple word like big [bɪɡ]. The nucleus is the most sonorous part, so in this word, the vowel [ɪ] is the nucleus. The consonant that comes after the vowel nucleus [ɡ] is the coda, and the consonant that comes before [b] is the onset. The only part of a syllable that always has to be there is the nucleus. Some syllables have an onset but no coda, like the word day [deɪ], and some syllables have a coda but not onset, like the word eat [it]. And the occasional syllable has neither an onset nor a coda, just a nucleus, like the word I [aɪ]!
What about a single-syllable word that has more consonants in it? Let’s look at screens. Again, the vowel [i] is the nucleus of this syllable, and the consonants [nz] that come after the nucleus form the coda. There are three consonants [skɹ] before the nucleus, and they form the onset. When there’s a group of consonants in the onset or coda we call them a cluster.
Monosyllabic words are pretty straightforward. How does it work with words that have more than one syllable, like raptor? It’s got two syllables, so it has two nuclei [æ] [ə]. The consonant at the beginning of the word [ɹ] is the onset of the first syllable, and the consonant at the end of the word [ɹ] is obviously the coda of the second syllable. What about these two consonants in the middle? In the word raptor, the [p] is the coda of the first syllable and [t] is the onset of the second syllable, but there are other logical possibilities. We could just as easily say that the first syllable has a coda cluster [pt], or that the second syllable has an onset cluster [pt]. How does the mental grammar organize consonants in the middle of a multi-syllabic word?
Well, it’s not random, and the mental grammar doesn’t just try to distribute consonants evenly. There’s a systematic principle that operates in the mental grammar, which is that onsets are greedy. To see what that means, let’s look at a word that has a bunch of consonants in the middle, like emblem. There are three consonants [mbl] in the middle of this word, so there are four logical possibilities for how they could be organized. It could be that all the consonants go in the onset of the second syllable. It could be that they all go in the coda of the first syllable, or they could be divided up between the coda of the first and the onset of the second, with a couple of possible permutations. What does the mental grammar do with these consonants?
The principle that onsets are greedy means that an onset will take as many consonants as it can. So this first option here has the greediest onset: it has the greatest number of consonants in an onset position. But it looks pretty weird, doesn’t it, to have a syllable start with [mbl]? A greedy onset takes as many consonants as it can within the grammar of that language. It’s a principle of English grammar that words don’t begin with a cluster like [mbl], and neither do syllables. Of these four options, the one that has the greediest onset that is possible within English is this one: the [m] is the coda of the first syllable, and the consonant cluster [bl] is the onset of the second syllable.
Let’s look at one more example to illustrate this idea that onsets are greedy. Consider the word ugly. The two vowels [ʌ] [i] form the two nuclei of the syllables; there’s no onset for the first syllable, and no coda for the second syllable. So there are three logical possibilities for these middle consonants [ɡl] — they could both be the coda; they could both be the onset; or they could split the difference. Which does the mental grammar do? The onset is greedy, so it wants to take as many consonants as it can. We know that [ɡl] is a possible onset in English, because there are lots of words that start with [ɡl], like glue, glass, glamour. So because [ɡl] is a possible, grammatical onset cluster in English, the onset of the second syllable takes all of it, and leaves no consonants in the coda of the first syllable.
Let’s sum up. Syllables are units within words, and they also have an inner structure of their own. Every syllable has a nucleus, which is the most sonorous part of the syllable: a vowel or another sonorous sound. If there are consonants, which are less sonorous, they make up the onset and coda of the syllable. And in the middle of a word, onsets are greedy: they’ll take as many consonants as they can, within the constraints of the grammar of the language.