When both or all participants in a conversation understand both languages, switching has a purpose. It's almost as if a third language is introduced. Codeswitching may occur in large blocks of speech, between sentences or within sentences.
Some authors have made a distinction between language mixing and codeswitching. Language mixing has been used to refer to early bilingual infants who sometimes seem (on the surface) to use either language indiscriminately. Codeswitching was then used to refer to bilinguals who had separated their two languages. Many authors now feel that a distinction between mixing and codeswitching is not sensible or real. This discussion relates to another vigorous debate about whether a young bilingual child develops two separate linguistic systems or just one integrated system. While this issue has not been resolved by research, there is a growing belief that bilingual children (as early as two years old) notice two different languages inputs depending on who is talking, where and when. Very early on, young children know which language to speak to whom in what situation.
Monolinguals who hear bilinguals codeswitch may believe that it shows a deficit, or a lack of competence in both languages. Bilinguals themselves may be anxious or apologetic about their codeswitching and attribute it to sloppy language habits. Few bilinguals keep their two languages completely separate. Few bilinguals speak both their languages with native speaker fluency. One language may influence the other, and sometimes the bilingual's dominant language influences his or her less dominant language. However, codeswitching is a valuable and purposeful communication strategy. It does not happen at random. There is usually considerable reason and logic in changing languages.
Children tend to codeswitch only when they are talking to people who understand both languages. Also children soon become aware if codeswitching is acceptable or not with different people. That is, bilinguals quickly learn to recognize those social situations and those people with whom they can and cannot codeswitch.