= Monophthong
A <monophthong> is a <vowel> with relatively stable quality through its central portion, in contrast with a <diphthong>'s substantial movement between qualities. Relatively stable does not mean physically unchanging: neighboring sounds, <vocal tract> movement and the analysis interval can all affect a recording.<Essentials of Linguistics (2nd edition) by Catherine Anderson and others>[§3.5, Multiple vowel qualities in sequence]{ref}
For a stipulated acoustic illustration, hold the first two <formants> at $F_1=300$ and $F_2=2200$ <hertz>, while varying <fundamental frequency> from 100 to 150 <hertz>. The model changes voice <pitch> without requiring the same change in the formant pattern. Conversely, changing <formants> can change vowel quality while <fundamental frequency> stays fixed.
An experimental analysis can record a sustained <vowel> and compare several points within a consistently chosen interval. Changes concentrated near consonant boundaries need not make the whole segment a <diphthong>. The distinction therefore combines a phonetic trajectory with an analysis of the language's sound units.
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