Another peculiarity of inflecton, in contrast with word formation, is the rigid parallelism of underlying and resultant forms. Thus, nearly all English singular nouns underlie a derived plural noun and, vice versa, all English plurals nouns are derived from a singular noun. [...] Given one of these, the speaker is usually capable of producing the other. Each such set of forms is called 'paradigmatic set' or 'paradigm' [...]
- Bloomfield (1935), Pag. 223