[…] 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), Pag. 65