In the phonetic structure of these languages [Scythian], the most striking trait is the so-called “harmonic sequence of vowels” […] and it is the general law that the vowels of the various endings shall be of the class of that in the root, or in its last syllable- thus marking the appurtenance and dependency of the endings in their relation to the root in a manner which, though undoubtedly at first euphonic only (like the German ‘umlaut’), has lent itself usefully to the purposes of formal distinction.
- Whitney (1875), a pag.234