Overt morphemes are explicit not zeroed morphemes building up words. Covert morphemes are implicit morphemes, i.d. a morpheme having no explicit representation in the actual expression (a contrastive absence of morpheme expressing a certain function). The notion of covert morpheme coincides with the notion of zero morpheme.
Example:
dogs – two morphemes, both overt: one lexical (root) and one grammatical expressing the plural.
dog – two morphemes, the overt root and the covert grammatical suffix of the singular.
III) according to segmental relation (ñåãìåíòíîå îòíîøåíèå)