Fromkin's Introduction to Linguistics says:
We classify vowels according to three questions:
- How high or low in the mouth is the tongue?
- How forward or backward in the mouth is the tongue?
- Are the lips rounded (pursed) or spread?
Merriam Webster says:
purse
transitive verb
1:
to put into a purse
2:
PUCKER, KNIT
pursed his brow
How are pursed and rounded synonymous?