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.^[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.