The consonant can be a stop, nasal, or semivowel (that is, do not occur initially).
en.wikipedia.org When two vowels come together, they are either separated by a glottal stop, fuse into a single vowel, or the first vowel reduces to a semivowel.
en.wikipedia.org After two consonants, the latter being a semivowel, all vowels are long.
en.wikipedia.org It is not often clear, however, whether such sequences involve a semivowel (a consonant) or a diphthong (a vowel), and in many cases that may not be a meaningful distinction.
en.wikipedia.org Most importantly, diphthongs are fully contained in the syllable nucleus while a semivowel or glide is restricted to the syllable boundaries (either the onset or the coda).
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