Richard Wordingham wrote:
> 4 vowel systems are usually more asymmetric. For yet more examples, we
> have the
> Proto-Austronesian system of /a, e, i, u/...
I'm sure you're aware of this, but others may not be: PAN *e represents
central schwa, not a mid/low front vowel. (At least, last I heard...) Its
use dates from days of the typewriter, when inserting a real schwa into your
ms. was laborious and prone to omission, no longer a problem thanks to
computers, except in email. But the system was indeed asymmetric, and *e was
somewhat restricted as to occurrence in the proto-language, probably was
always unstressed, and shows many peculiarites in later development.