The absence of any distinction whatsoever among degrees of vocalic aperture should be quite exceptional; two degrees are well attested ( in classical arabic, for instance) though not frequent; three degrees are, probably, to be considered a norm; four degrees are not rare, but hardly very stable […] more than four degrees are not likely to endure as such, neighbouring units being apt, in such pattern, to become distinct by means of some other difference than sheer aperture.
- Martinet (1962), a pag.79-80