Complete overlapping, which associates a segmental element in one environment sometimes with one phoneme and sometimes with another […] conflicts with the one-one requirement of phonemics. E.g. if some occurrences of [K] in [s−u] were /k/ and others were /g/, we could hear the segments [sku] and not know whether to write it /sku/ or /sgu/.
- Harris (1951), a pag.65, n.14

[…] the classes of 15.4 [Alternative Procedure: Classes of Morpheme-in-Environment], and any morpheme classification used for chapter 16 [Morpheme Sequences], are classes of morphemes-in environments, so that a single morpheme or morphemic sequence (or in any case a single phonemic sequence) may in one environment be a member of one class and in another environment a member of another class. This is only partial overlapping of morpheme classes, and given two different utterances containing the same morpheme we can tell to which class the morpheme belongs in each utterance by noting the different environments. In the case of 'I can tell my horse’s' (or: 'horse is) running' […], there is a segment /Əz/, member of a morpheme {’s}, member of class 'Na', and a segment /Əz/, member of a morpheme {be}, member of class 'V'. However, here we have complete overlapping, and we cannot tell which morpheme, of which class, occurs because the environment is identical.
- Harris (1951), a pag.271, n.24