Principle 2. Forms which possess a common semantic distinctiveness but which differ in phonemic form (i.e. the phonemes or order of the phonemes) may constitute a morpheme provided the distribution of formal differences is phonologically definable. [...] the difference of distribution between the alternant forms of a single morpheme is phonologically definable.
- Nida (1949), a pag.14

If we can define the distrubution of alternants in terms of the phonological situations in which they occur, then we may combine such forms as allomorphs of the same morpheme, provided, of course, that they fullfill the requirement of 'common semantic distinctiveness'.
- Nida (1949), a pag.16