The alternation between ‘a’ and ‘an’ is therefore said to be ‘phonemically conditioned’. On the other hand, the alternation between /wàjf/ (in the singular ‘wife’) and /wàjv/ (in ‘wives’) for the morpheme ‘wife’ is ‘morphemically conditioned’. [...] Morphemically conditioned alternation is always non-automatic; automatic alternation is always phonemically conditioned. There are also phonemically conditioned alternations which are not automatic. The English ‘a’ / ‘an’ alternation is of this sort.
- Hockett (1958), a pag.281