Try the following:
We can define the vowels of human language with these three parameters:Say the vowels in FLEECE and PALM in quick succession: ee, ah, ee, ah. What is your tongue doing? The FLEECE vowels is high front, and the PALM vowel is low back.Say the vowels in GOOSE and PALM in quick succession: oo, ah, oo, ah. Now what's happening? What are your lips doing? The GOOSE vowel is rounded, and the PALM vowel is unrounded.Finally, say the vowels in FLEECE and GOOSE in quick succession: ee,oo, ee, oo. This time, two things happen: The FLEECE vowel is front, unrounded; the GOOSE vowel is back, rounded.
Front-back
High-low
Rounded-unrounded