A subsidiary [...] grammatical process is that of internal vocalic or consonantal change. In some languages [...] the former of these has become one of the major methods of indicating fundamental changes of grammatical functions.
- Sapir (1921), a pag.73

Vocalic change is of great significance [...] in a number of American Indian languages. In the Athabaskan group many verbs change the quality or quantity of the vowel of the radical element as it changes its tense or mood.[...] In another Indian language, Yokuts, vocalic modifications affect both noun and verb forms.
- Sapir (1921), a pag.74

Consonantal change as a functional process is probably far less common than vocalic modifications. [...] in English, certain nouns and corresponding verbs differ solely in that the final consonant is voiceless or voiced [...]
- Sapir (1921), a pag.74