[...] in (American) English there is a perceptible difference in the length of the vowel 'a' of 'bad' and 'bat' [...]. The alternations are governed by mechanical considerations [...]. They take care of themselves [...], and it is not always easy to convince natives of their objective reality, however sensitive they may be to violations of teh unconscious rule in the speech of foreigners.
- Sapir (1925), Pag. 37