A slightly more extensive type of assimilation occurs in the series 'comparable', 'context', 'congregate' [...] Though this prefix has three forms /kam-, kan-, kaŋ/, their distribution can be phonologically defined [...] We may call these forms morphemic alternants or allomorphs.
- Nida (1949), Pag. 14

There is absolutely no limits to the degree of phonological difference between allomorphs. Allomorphs may consist of very different phonemes as in the plural formatives /(-əz ~ -z ~ -s) ∞ -ən ∞ -0/ or they may be quite similar.
- Nida (1949), Pag. 44

Allomorphs are complementary in all occurrences, but morphemes may be complementary only in certain structural series.
- Nida (1949), Pag. 144