Diphthongs

Diphthong is a gliding monosyllabic speech sound that starts at (or near) an articulatory position for one vowel and moves towards a position for another. The diphthong's dynamic quality is in contrast to the vowels which at least in theory are stable segments and can thus be termed monophthongs.
The diphthongs are produced by varying smoothly the vocal tract between one vowel configuration to another's. The figure bellow shows a spectrogram plot for a the diphthong /a^y/ spoken by a male talker (myself). In the picture it's possible to see the gliding motion of the formants.

According to the diphthong's definition given above, there are six diphthongs in American English, namely /a^y/ (as in buy), /a^w/ (as in down), /e^y/ (as in bait), and /c^y/ (as in boy), /o/ (as in boat), /ju/ (as in you).