Diphthong
This is the phonetic sound that results when two vowels are next to each other: “ai” and “ea” and “ou,” for example.
It’s pronounced “diff-thong.”
Why I Looked It Up
I saw a graphic about sounds gone from our alphabet: 12 Symbols From Our Alphabet No Longer Used Today
One of them was the diphthong you still see in Scandinavia where an “a” and an “e” are pushed right up against each other.
It stuck me that I didn’t even know what a diphthong was.
Postscript
Added on
I took a screencap of an article, but I can’t remember where it was from. It says, in part:
Try saying “say” or “way” very slowly, drawing out the vowel at the end. Notice that as you say “aaaaaay”, your tongue movies. This is because “ay” is secretly not one but two vowels said in quick successful. (Linquists call such double vowels “diphthongs.”)
Postscript
Added on
In Hidden Potential, there’s a footnote after the phrase “The New Encyclopaedia Brittanica.”
Tradition demanded that, new or not, the book still be spelled using the diphthong…
I assume they were referring to the “ae” together.