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
Iu Mien language represented in the Iu Mien United Script using combinations of the six letters, , , , , , and . According to Bruhn, the monophthongs are , , , , , , , , , and . The diphthongs are , , , , , , , . Furthermore, additional diphthongs and triphthongs can be formed from the aforementioned vowels through /i/- or /u/-on-gliding (having /i/ or /u/ before the vowel). Such vowels attested by Bruhn include , , , , , , , , , , , , , , , , , , , , and . The