From the list of morpheme alternants which results from the preceding step, we take any two or more alternants which have what we consider the same meaning (but different phonemes) and no one of which ever occurs in the same environment as the others. The two or more alternants which meet these conditions are grouped together into a single MORPHEME UNIT; 'am', which occurs only in phrases with 'I', and 'are', which never occurs with 'I', are put into one morpheme unit. In many cases when we take one alternant and try to find another to group with it, we fail: e.g. in the case of 'walk', 'rain'. In such cases we say that the single alternant constitutes a morpheme unit by itself. A morpheme unit is thus a group of one or more alternants which have the same meaning and complementary distribution […]. The case is different with 'twenty' and 'score', even though they have the same meaning and never occur in the same environment. For there is no morpheme unit in English which consists of only one alternant and which occurs in the combined distribution of 'twenty' and 'score'. Therefore, we consider the alternants 'am', 'are', 'be' as being members of a single morpheme unit; but of the alternants 'twenty' and 'score', each constitutes a morpheme unit by itself.
- Harris (1942a), a pag.171