Bi- or multilingual speakers sometimes engage in speech using several languages at their disposal. In this case study, codeswitching – the alternation of languages – in the L2 classroom is investigated in the context of a Swedish upper secondary school teacher's practice.

Four lessons were observed, and the teacher was interviewed.

Four types of teacher codeswitching were identified, and motivated by the teacher due to the: relation to individual students, knowledge of student proficiency, level of topic complexity, and level of affect.

In conclusion, the teacher codeswitches to achieve social, pedagogical, and affective goals, among other reasons, to motivate, clarify, and reprimand. The concept of a "complexity threshold" and its possible implications for teachers and students are discussed, as well as the quality of classroom discourse with suggestions for further research of potential educational affordances arising from codeswitching.