[...] there are two logically possible elementary ways in which linguistic forms can differ from each other: quantitatively and qualitatively. Qualitative differences may stem either from differences in sound properties or from differences in the temporal relations of sound properties. Thus, for example, the two sound strings "pa" and "ba" differ from each other in some of the sound properties that are involved in their pronunciations.
- Moravcsik (2004), a pag.310