I always knew about diphthongs, but I didn’t know there were monophthongs and triphthongs as well. Monophthongs may be obvious [single, consistent vowel sounds], but triphthongs — single-syllable vowel sounds that vary three times over their duration — are more elusive. The only English example I have found is the pronunciation of “our” in dialects that drop the “r,” like a Bostonian accent, I guess, where someone would say “owww-uhhh-aaaaah,” to put it exaggeratedly.
Wow, I just greatly expanded the number of English words with the -phth- compound in them that I know. Monophthong, diphthong, triphthong, ophthamology [and variants], exophthalamos [and variants], naphthalene [and variants]. Man, that combination of letters together just looks odd.