The term code-switching (or, as it is sometimes written, codeswitching or code switching) is widely discussed and used in linguistics. The most common definition of code-switching is using two or more language varieties in conversation.
Concerning code-switching, the active research continues for about half century. During this time it has been developed into an independent linguistic discipline. This fact is explained by the increased interest in the difficulties of language contact.
A pivotal thing for understanding the nature of various types of code-switching is K. Myers-Scotton’s opposition of ‘marked’ and ‘unmarked choice’ in code-switching. When the speaker follows the established rules of speech behavior in the language community and makes switching in accordance with the expectations of the listener is called ‘unmarked code-switching’. But if the speaker consciously produces switching in a way that it is noticed as a deviation by the interlocutor it is called ‘marked code-switching’ .
To the latter position is quite close Peter Auer’s view where he proposed to distinguish cases of a combination of two languages from code-switching in which their usage is not local for speakers but it possesses rather global meaning, i.e. is not determined by the specific features of the situation and that such use of languages is accepted in this community .
Milroy and Muysken (1995) suggests “perhaps the central issue in bilingualism research is code-switching” [3, p.7]. In fact, regarding the studies on bilingual speech, it can be proved that bilinguals have a tendency to mix their languages in a process of communication. Consequently, such mixing has been evoked misinterpretations as bilinguals do not possess enough competence in communicating one or both languages.
According to Chirsheva, in multilingual world, interplay of languages, bilingualism and bilingual type of communication are becoming a widespread phenomenon [4, p.23]. It is believed that a person who speaks two languages is able to participate in two monolingual conversations as well as in bilingual types of communication. A characterization of bilingual type of communication is interplay of languages which include forms such as interference and code-switching. Bilingual type of communication is described with incomplete deactivation of one languages and with gradation of activity of two languages in speech of bilingual while producing an utterance. In a production of speech, a person who possesses two languages may deactivate first language incompletely which provides an interference in speech in second language which mostly occur in monolingual type of communication. When a bilingual is having a conversation with other bilinguals, it can be seen gradation of activity of two languages which is called bilingual speech.
Muhamedova and Auer assume that languages have a potential to be mixed in such a way that language A is considered as a dominant language and language B is an embedded language [5, p.35]. Embedded language can be inserted in the forms of phrases or even single words into the grammatical frame determined by language A (matrix language). The grammatical frame of the sentence is provided by the grammar of language A (matrix language) whereas language B (embedded language) is utilized only in complex insertions in order to define the structure of inserted constituent.
Code-switching examines ways of dealing with the difficulties and language barriers occurring while sentence-planning by using elements of two or more languages. This phenomenon particularly noticed in child’s speech when they make errors and have some problems in acquisition of phrases or sentences. From the psycholinguistic point of view, one of the crucial feature of code-switching is flexibility of language production. Psycholinguists consider the problem for debate called as ‘co-ordination problem’ which means the fluent achievement of speech.
In terms of language contact, code-switching or language alternation is a transition of the speaker from one language to another in the process of speech communication. A similar pattern is observed in societies where two (or more) languages are used rather than one. It is believed that bilinguals resort to code-switching because they can not express their thoughts in one language. To some extent, it is a truth and inability to transfer what they want to say force them to switch to another language in order to compensate those omissions.
According to the researchers, it is in bilingual (multilingual) language teams native speakers receive an opportunity of contrastive based on intuitive deductions contradistinction of two different language systems. Bilinguals, i.e. people who speak two (or more) languages usually “distribute” their usage depending on the conditions of communication, for instance, in official meetings where dominant language is only one while communicating with the authorities but in everyday life (in the family, contacts with neighbors) using several languages simultaneously is acceptable.
Code-switching can be induced, for instance, by the change of addressee, i.e. the one to whom the speaker addresses. If the addressee speaks only one of the two languages that the speaker knows, then, the latter, no doubt, has to utilize a language which is familiar to the addressee. Although, until this moment in communication with bilingual interlocutors another or both languages could be used. Switching to a known language code can occur even in the case of changing group members. For example, if a third person owning only one language joins to the conversation of two bilinguals and that language known to these bilinguals, they will speak in the language of the third person. Refusal of interlocutors to switch to the code familiar to the third participant of communication can be regarded as an unwillingness to involve him to a conversation or as a neglect to his communicative inquiries.
The factor that evokes code-switching can be a change of a role of the speaker. Take, for example, the role of a father (when communicating in the family) he can use a convenient code for him but when turning to the authorities, he forced to switch to more or less conventional forms of speech.
The topic of conversation also influences on a choice of code. Researchers who dealt with the problems of communication found out that members of a certain sphere have discussions in a language that has the appropriate special terminology which describes technical processes, devices, instruments, etc. But as soon as a change of a theme occurs from production to household, another language code switches on: native language or dialect of interlocutors. In a monolingual society, with such a change of code there is a switch only from professional language to vernacular language means.
Code-switching can be used by bilinguals as a “sociolinguistic tool” or “technique” .
Code-switching can serve for the expression of speaker’s solidarity from a certain social group. When a listener responds to a code-switching with the same switch, a specific connection is established between the speaker and the listener. At the same time, exclusion of unwanted listeners (who do not speak the language to which interlocutors switch) can happen. Sometimes a code-switching is used to show the attitude of the speaker to the listener. While monolinguals express their relation by varying the content of speech, bilingual speakers often use a code-switching for this purpose.
Code-switching may also serve to impress a listener. If bilingual interlocutors are used to communicate in a certain language, switching from one language to another is unexpected for the interlocutor and evokes a “special effect”. In other words, a code-switching is not only a linguistic but also a sociolinguistic phenomenon .
As a rule, the situation of full bilingualism in the language team has two immensely opposite consequences. On the one hand, as it has mentioned above, native speakers have a possibility to make a contradistinction of two systems and, as a result, understanding of structural differences. On the other hand, constant use of two languages simultaneously expressed in permanent switching of codes, native speakers cease to distinguish codes which they use  and there is a possibility of interference. Therefore, it should be noted that code-switching and interference have an absolutely difference which is that interference implies modification of grammatical, syntactical or phonetic forms of one language under the influence of another but not the change of one language to another. Furthermore, code-switching contributes to the implementation of the act of communication whereas interference indicates mixed usage of different language norms or rules which can lead to misunderstanding. Interference mainly depicts the relationship between language systems when they are in contact and code-switching has to do with bilingualism itself. Thus, it can be concluded that interference may manifest in speech of bilingual along with code-switching. However, it is vital to distinguish described phenomena .
It should be noted that there are three main categories in the study of the problem of code-switching in modern researches. To the first category belong researchers who investigate “extrasentential” (or “intersentential”) code-switching. In this way, some representatives of theoretical linguistics and psycholinguistics conduct their research. According to N. Kamwangamalu, representatives of theoretical linguistics primarily examine grammatical aspects of code-switching . Psycholinguists are interested in how sentences are generated from code-switching, is there a difference between sentence construction of monolingual and bilingual speakers and the number of grammatical systems that are included in a sentence with code-switching .
The second category includes scientists who follow sociolinguistics. They are not so much interested in a difference between “intra-sentential” and “intersentential” code-switchings as in finding out the reason for the existence of such phenomenon code-switching among bilinguals.
The third category entails linguists who conduct researches on code-switching in the frame of communicative and pragmatic approach. P. Angermeyer claims that representatives of this category aim to scrutinize the structure of communication process in which a phenomenon of code-switching takes place. Moreover, he attempts to figure out the role of switching codes in establishing the order of replicas of interlocutors, thematic construction of communication process, starting a conversation and continuing a talk on a certain topic, switching from one topic to another, etc. .
What are the parts of sentences where switchings take place? It depends on the nature of impact of those factors just discussed above. If the speaker can foresee and even plan the influence of a particular factor, then the switching occurs naturally, i.e. at the end of the phrase with the most quite mode of communication – at the end of the discussion of any topic. However, if the intervention of the factor inducing code-switching is unexpected for the speaker, he can switch from code to code in the middle of the phrase sometimes even without saying a word. In case of high level proficiency of codes or sub-codes, the process of code-switching may not be realized by the speaker if their use is highly automated. Especially in cases where another code is not entirely used but in fragments. For instance, a person can insert elements of another language into his speech such as idioms, modal words, interjections, particles.