= 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.<Essentials of Linguistics (2nd edition) by Catherine Anderson and others>[§3.5, Multiple vowel qualities in sequence]{ref}
A recording can represent that change through <formant> trajectories. For an invented illustration, let normalized vowel time be $0\le u\le1$ and set
$$
F_1(u)=700-350u,\qquad F_2(u)=1000+1200u
$$
in <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.
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