In describing and identifying morphemic alternants (allomorphs) it is frequently convenient to distinguish two types: basic and non basic. This distinction may include phonologically defined allomorphs or morphologically defined allomorphs or both. In the series /-əz ~ -z ~ -s/, whose distribution is phonologically definable, it is possible to select one alternant as basic and the others as phonologically "derived". [...] If two alternants have approximately the same frequency of occurrence, but if one of them combines usually with forms that exhibit considerable modification of their underlying forms, while the other combines with unmodified forms, the latter is likely to illustrate the more productive pattern, and - other things being equal - should be chosen as the basic alternant.
- Nida (2004), a pag.51-52