Vowel whose quality does not change over the duration of the vowel is called a monophthong. Monophthongs are sometimes called "pure" or "stable" vowels. A vowel sound that glides from one quality to another is called a diphthong, and a vowel sound that glides successively through three qualities is a triphthong. All languages have monophthongs and many languages have diphthongs, but triphthongs or vowel sounds with even more target qualities are relatively rare cross-linguistically. English has all three types: the vowel sound in "hit" is a monophthong , the vowel sound in "boy" is in most dialects a diphthong , and
Vietnamese phonology Any consonant may occur in as an onset with the following exceptions: w: the onglide (sometimes transcribed instead as labialization on a preceding consonant): V: The vowel nucleus V may be any of the following 14 monophthongs or diphthongs: . G: The offglide may be or . Together, V and G must form one of the diphthongs or triphthongs listed in the section on Vowels. C: The optional coda C is restricted to labial, coronal, and velar stops and nasals , which cannot cooccur with the offglides . T: Syllables are spoken with an inherent tone contour: Below is a