【正确答案】正确答案：When pure or monophthongs are pronounced, the quality remains constant throughout the articulatioin. In contrast, those where there is an audible change of quality are called vowel glides. Vowel glides are produced by moving from one vowel position to another through intervening positions. If a single movement of the tongue is involved, the glides are called diphthongs. A double movement produces triphthongs.