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

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

In the Celtic languages the initial consonants undergo several types of change according to the grammatical relation that subsists between the word itself and the preceding word.
- Sapir (1921), a pag.75