It is empirically discoverable that in all languages which have been described we can find some part of one utterance which will be similar to a part of some other utterance. ‘Similar’ here means not physically identical but substitutable without obtaining a change in response from native speakers who hear the utterance before and after the substitution: e.g the last part of 'He’s in.' is substitutable for the last part of 'That’s my pin.'
- Harris (1951), a pag.20

It is not necessary for the class members to be ‘similar’. I.e. for the class to be distinguished by any feature other than that in respect to which the class was set up. E.g. quite distinct segments amy br grouped into the phoneme /t/; highly dissimilar morphemic segments are grouped into the morpheme {be}; and there is no formal similarity among the morpheme sequences which are included in the class 'N'.
- Harris (1951), a pag.367, n.9