The vowels are usually separated into two groups: monophthongs and diphthongs. Monophthongs are manufactured with a single articulatory movement (that's why the prefix mono), when diphthongs feature an articulatory alter. Diphthongs have two constituents: a glide (or even a semivowel) plus a monophthong. You can find some disagreement about exactly https://erickw5162.blogstival.com/50489489/the-best-side-of-korean