Each type is categorically distinct. Cardinal vowels of Daniel Jones Vowels repesented as points in a quadrilateral that represent the positions of the highest point of the tongue during the production of the vowel. Vowels were assumed to be spaced at auditorily equal intervals of tongue position. Sir Isaac Newton recognized the relation between vowel qualities and resonances he noted that he could hear a progression of different vowels as he poured beer into a flaggon.
Individuals differ in range of formant frequencies.
But , rounding effects formant frequencies. So position within the formant graph is not independent of rounding. Note that formant frequency difference between front and back vowels is maximized when back vowels are rounded and front vowels are unrounded. Same tongue shape, same position on cardinal vowel chart are associated with different formant frequencies. Tongue will expand out in other dimensions to conserve volume.