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
Khmer language in pronunciation between individual speakers, even within a dialectal region. The description below follows Huffman (1970). The number of vowel nuclei and their values vary between dialects; differences exist even between the Standard Khmer system and that of the Battambang dialect on which the standard is based. In addition, some diphthongs and triphthongs are analyzed as a vowel nucleus plus a semivowel ( or ) coda because they can not be followed by a final consonant. These include: (with short monophthongs) , , , , ; (with long monophthongs) , ; (with long diphthongs) , , , , and .