Alternations are also widely spread on the synchronic level in the present-day English and are known as contextual. In connection with contextual sound alternations there arises a problem of phonemic identification of alternated sounds. The study of the relationship between phonemes and morphemes is called morphophonemics. The interrelation of phonology and morphology is also known as morphophonology оr mоrрhоnоlogy which is actually the phonology of morphemes. Morphonology studies the way in which sounds can alternate in different realizations of one and the same morpheme.
We are interested in the sound in its weak position. Scholars of different trends are not unanimous in solving the problem.
The so-called morphological (Moscow phonological) school supports the theory of neutralization of phonemes. The concept of neutralization derives originally from the Prague School of phonology. Neutralization occurs when two or more closely related sounds, which are in contrast with each other in most positions, are found to be non-contrastive in certain other positions. That means that there are environment where the two sounds do not contrast with each other, even though they normally do. When this happens, the opposition between the two sounds is said to be neutralized. The loss of one or more distinctive feature(s) of a phoneme in the weak position is called phonemic neutralization.
The Moscow philologists claim that interchange of sounds manifests close connection between phonetics and morphology. Alternations are observed in one and the same morpheme and actualize the phonemic structure of the morpheme. Thus, phonemic content of the morpheme/is constant. It should be noted here that alternations of morphemes cannot be mistaken for the oppositions of minimal pairs in different stems of words. Lets us compare some examples: postman [ə] < [æ], sixpence [ə] < [e]. Thus, one and the same sound may belong to different phonemes
The supporters of the morphological trend define the phoneme as follows «Это функциональная единица, представленная рядом позиционно чередующихся звуков» (М.В. Панов). The notion of «фонетический ряд», suggested by R.I. Avanesov, demonstrates positionally determined realizations of the phoneme. Positionally alternating sounds are grouped into one phoneme whether they are similar or have common features (that is common allophones) with other phonemes.
The Russian preposition с + noun may have the following realizations: с Колей — [c], с Тимошей — [c'], с Галей — [з], с Димой — [з'], с Шypoй — [ш]. с Женей — [ж], с Щукарём — [ш'].
In the morphological conception the alternations of the phonemes are not analyzed apart from the morpheme, as form and content make dialectical unity. The phonetic system is not isolated from the grammatical and lexical structure of the language, and the unity between the form and the content cannot be destroyed.
Yet as an answer to the problem is not entirely satisfactory since ordinary speakers are in no doubt that the sound which occurs in a word like гриб is [п] not [б], and in English word speak [ph] is nothing but [p]. The perception of the listeners makes us find the morphological conception too discrepant and confiding.
The so-called Leningrad (Petersburg) school asserts that the phoneme is independent of the morpheme. The supporters of this conception claim that the phoneme cannot lose any of its distinctive features. In the line of words of the same root morpheme (гриб - грибы) the sound [п] is an allophone of the phoneme /п/ and the sound [б] manifests the phoneme /б/. Consequently, the consonants  and [п] do not lose any their distinctive features and represent different phonemes. It seems that according to this point of view the unity between the form and the content is destroyed, thus phonology is isolated from morphology.
According to N.S. Trubetzkoy, an archiphoneme is defined as a combination of distinctive features common to two phonemes. It consists of the shared features of two or more closely related phonemes but excludes the feature which distinguishes them. For example: archiphoneme [П] consists of the features: bilabial, plosive, but excludes voicing which separates them.
One of the disadvantages in extending the notion of an archiphoneme is that the Prague School phonologists limited neutralization to closely related phonemes. A neutralization can be said to occur only if there is uncertainty about the identity of the sound in the position of neutralization. Before two phonemes can be neutralized, they must have common qualities which do not occur in other phonemes. Thus [p], [b] can neutralize because they are the only labial plosives in the language, they share these two features, but no other sounds share them. However, [n] and [ŋ] cannot neutralize, so any neutralization of nasals must involve all the three of them - [n], [ŋ], [m].