Diphthong

A diphthong is a vowel whose quality changes during its production, conventionally treated as one dynamic vowel unit. A monophthong has relatively stable quality instead. The vowel in English toy, often transcribed [ɔɪ], moves from a back rounded quality toward a high front unrounded quality.^[ref]

A recording can represent that change through formant trajectories. For an invented illustration, let normalized vowel time be $0\le u\le 1$ and setin hertz. These are stipulated linear interpolation tracks, not measurements of toy. Their endpoints differ even if fundamental frequency stays fixed, separating changing vowel quality from voice pitch. Real trajectories need not be linear, and identifying a single diphthong rather than a sequence of separate vowels requires phonological evidence as well as an acoustic curve.