'An overt formal difference' means a contrast which is indicated by differences 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. The contrast between the singular 'sheep' /šiyp/ and the plural 'sheep' /šiyp/ consists of a zero and is covert. - Nida (1949), a pag.54