The second type of variation is common to all normal speakers of the language and is dependent on the phonetic conditions in which the fundamental sound [...] occurs. In most languages, what is felt by the speakers to be the 'same' sound has perceptibly different forms as these conditions vary. Thus, in (American) English there is a perceptible difference in the length of the vowel 'a' of 'bad' and 'bat', the 'a-'vowel illustrated by these words being long or half-long before voiced consonants and all continuants, whether voiced or unvoiced, but short before voiceless stops.
- Sapir (1925), a pag.37