In phonetics, what distinguishes true diphthongs?

True diphthongs are characterized by a noticeable change in the articulation of the vowel sounds within the same syllable, which means the mouth positions of the two vowels are typically far apart. This shift often involves a movement from one vowel sound to another, creating a smooth gliding motion during pronunciation. For example, in the diphthong "ai" as in "fine," the mouth starts in one vowel position and glides to another, which illustrates this significant positional difference.

Understanding why the other options do not accurately describe true diphthongs is also valuable. If the mouth positions were the same or advanced, it would not reflect the distinctive quality that defines a diphthong. Additionally, since true diphthongs are made up of vowel sounds rather than syllabic consonants, the option regarding syllabic consonants is not relevant in this context. Thus, the essence of true diphthongs lies in the combination of two distinct vowel sounds with markedly different mouth positions.