The aim of the phonological analysis is, firstly, to determine which differences of sounds are phonemic and which are non-phonemic and, secondly, to find the inventory of phonemes of the language.
As it was mentioned above, phonology has its own methods of investigation. Semantic method is applied for phonological analysis of both unknown languages and languages already described. The method is based on a phonemic rule that phonemes can distinguish words and morphemes when opposed to one another. It consists in systematic substitution of one sound for another in order to find out in which cases where the phonetic context remains the same such replacing leads to a change of meaning. This procedure is called the commutation test. It consists in finding minimal pairs of words and their grammatical forms. For example:
Minimal pairs are useful for establishing the phonemes of the language. Thus, a phoneme can only perform its distinctive function if it is opposed to another phoneme in the same position. Such an opposition is called phonological. Let us consider the classification of phonological oppositions worked out by N.S. Trubetzkoy. It is based on the number of distinctive articulatory features underlying the opposition.
1. If the opposition is based on a single difference in the articulation of two speech sounds, it is a single phonological opposition, e.g. [p]-[t], as in [pen]-[ten]; bilabial vs. forelingual, all the other features are the same.
2. If the sounds in distinctive opposition have two differences in their articulation, the opposition is double one, or a sum of two single oppositions, e.g. [p]-[d], as in [pen]-[den], 1) bilabial vs. forelingual 2) voiceless-fortis vs. voiced-lenis
3. If there are three articulatory differences, the opposition is triple one, or a sum of three single oppositions, e.g. [p]- [ð], as in [pei]-[ ðei]: 1) bilabial vs. forelingual, 2) occlusive vs. constrictive, 3) voiceless-fortis vs. voiced-lenis.
American descriptivists, whose most zealous representative is, perhaps, Zellig Harris, declare the distributional method to be the only scientific one. At the same time they declare the semantic method unscientific because they consider recourse to meaning external to linguistics. Descriptivists consider the phonemic analysis in terms of distribution. They consider it possible to discover the phonemes of a language by the rigid application of a distributional method. It means to group all the sounds pronounced by native speakers into phoneme according to the laws of phonemic and allophonic distribution:
1. Allophones of different phonemes occur in the same phonetic context. In this case their distribution is contrastive.
2. Allophones of the same phoneme(s) never occur in the same phonetic context. In this case their distribution is complementary.
There is, however, a third possibility, namely, that the sounds both occur in a language but the speakers are inconsistent in the way they use them, for example, калоши-галоши, and [‘ei∫э - ‘егжэ]. In such cases we must take them as free variants of a single phoneme. We could explain the case on the basis of sociolinguistics. Thus, there are three types of distribution: contrastive, complementary and free variation.