Principle 4. An overt formal difference in a structural series constitutes a morpheme if in any member of such a series, the overt formal difference and a zero structural difference are the only significant features for distinguishing a minimal unit of phonetic-semantic distinctiveness.
- Nida (1949), a pag.54

'An overt formal difference' means a contrast which is indicated by differencess in phonemes or in the order of phonemes. The distinction between 'foot' /fut/ and 'feet' /fiyt/ is an overt difference, since it consists in a difference of phonemes.
- Nida (1949), a pag.54