> > > vowels are denoted by diacritics on the basic consonant symbols:
> > > e.g. Ethiopic, Indic] and alphabets proper [with both vowels and
> > > consonants: e.g. Greek]).
> >
> > How do they justify the division between alphabets proper and abugidas
> > "syllabarys"?
>
> The obvious and sufficient justification is that it is convenient.

That's arbitrary.

> > Once you have established that e.g the vowels are
> > (consistently) diacritics, then why aren't they alphabets?
>
> Why aren't French diacritics (acute, grave, circumflex and diaeresis)
> reckoned as letters?

They do not consistently mark the syllable vowel (I mean that vowels
are indicated not only by means of these, also a, ou etc). If they
did, I would have that writing system and an alphabet in the same
class and not make a distinction because they happen to be written
as superscripts. If the prototype of alphabet is one phoneme <=> one
sign then why does it matter if some signs are written under, above,
next to etc.

> >That seems
> > to require that you have some objective difference between a sign
> > and a diacritic?
>
> .. a bunch of examples
>
> In short, the differences are practical.

But to classify them according to this difference is arbitrary as far
as I can tell. To say that e.g fully vowellized Arabic or Hebrew or
devanagari for Sanskrit are different from alphabets because you have
to read the signs in a snake-like direction is like saying boustrophedon
Greek is not alphabetic writing because suddenly you have read in
another direction?

best wishes

Harald