>Just liket that graded ablaut stuff, the actual 5 vowels are not
>observed in any langauge.
You can observe them in Sanskrit! Speaking very simply, some examples of
the following show all five PIE vowels::
(1) -a- which does not causes palatalisation (PIE *a)
(2) -a- which does cause palatalisation (PIE *e)
(3) -a:- lengthened in an open syllable without other cause: (PIE *o)
(4) -i- (PIE *i)
(5) -u- (PIE *u)
Peter