[…] where the phonetically trained investigator can hear two or three clearly distinct types of sound all representing one and the same phoneme in a given language, the different types are often called ‘allophones’. Thus we say that English /p/ is represented by at least two distinct allophones, an unaspirated [p] under certain environoing conditions, an aspirated /p’/ under others.
- Hockett (1958), a pag.63