[…] we frequently find morphemes whose distributions are partially identical: in some environments all of these morphemes occur, but in other environments only particular ones of these morphemes are to be found.
- Harris (1951), Pag. 243

We express the partial distributional identity of any two morpheme classes by saying that these two classes have a morphemic component in common. If classes 'A' and 'B' occur together in certain utterances while 'A' and 'C' do not, we may say that 'A' and 'B' each contain or represent a morphemic component which is not represented by 'C'. If 'A' and 'D' each occur in the environment '–B', whereas 'E' and 'G' do not occur there, we may say that 'A' and 'D', a well as the environment 'B', each represent a morphemic component which is not represented by 'E' and 'G' (or by 'C'). Many different cases of partial distributional identity among classes, i.e. many different conditions of limited co-occurrence or substitution, occur in various languages.
- Harris (1951), Pag. 301