We often find sets of morphemes all of whose members vary analogously in corresponding environments: e.g. {'knife'}, {'wolf'}, {'wife'}, all have only two members, the second differing only in having /v/ before {'s'} ‘plural’. In this case there are, however, other morphemes having the same phonemic form in the relevant respect (final /f/), but not having this alternation of members; {'fife'} has only one member in all environments ('fife', 'fifes').[…].
- Harris (1951), a pag.221