[…] along with the ‘f’ and ‘v’, as akin with them, especially in their dentilabial variety, we have the two English ‘th’-sounds, surd in ‘thin’ and sonant in ‘then’ […] real dentilinguals, produced between the tongue and teeth. These four, with the (German) ‘ch’-sound, we class as “spirants.” Historically, they have a special kinship in that they are all alike frequent products of the alteration of an aspirate mute; hence it is that they are so often, in various languages, written with ‘ph’, ‘th’, ‘ch’ (=‘kh’).
- Whitney (1875), a pag.65

If the lips are brought together in loose instead of close contact, and the breath forced out between them, there is heard an ‘f’-sound; or, if the breath be intonated, a ‘v’-sound. These, however, are not precisely our English or French (nor the general German) ‘f’ and ‘v’; for, in the latter, the tips of the teeth are brought forward and laid upon the lower lip, and the expulsion is made between them; giving a product somewhat differently shaded, a dentilabial instead of a pure labial sound.
- Whitney (1875), a pag.64