Morphophonemes
A morphophoneme is a theoretical unit at a deeper level of abstraction than traditional phonemes, and is taken to be a unit from which morphemes are built up. A morphophoneme within a morpheme can be expressed in different ways in different allomorphs of that morpheme (according to morphophonological rules). For example, the English plural morpheme -s appearing in words such as cats and dogs can be considered to consist of a single morphophoneme, which might be written (for example) //z// or |z|, and which is pronounced as after most voiceless consonants (as in cats) and in most other cases (as in dogs).
Read more about this topic: Phoneme