This paper reports on a study that investigated teacher code switching consistency and precision in multilingual secondary school mathematics classrooms in South Africa. Data was obtained through interviewing and observing five lessons of each of three mathematics teachers purposively selected from three township schools in the Eastern Cape Province. Elements of Gumperz and Mercer's work on lesson categories and Dowling's Domains of Mathematical Practice were used to analyse data. Results showed that code switching frequency in general was inconsistent across different lessons for the same teacher. Code switching frequency by all teachers was, however, consistently highest during questioning and explaining when teaching. All participating teachers used code switching strategies most consistently in the public domain and least consistently in the esoteric domain. Some formal isiXhosa translations of mathematical terms were consistently and precisely used and some were not. Two major forms of code switching emerged, namely borrowing code switching and transparent code switching. Very little transparent code switching, which is critical for supporting students' understanding and thinking in mathematics, was evident in teacher language. Teachers were consistent in the use of borrowed terms. We conclude that consensual understanding of best practices for code switching is required to promote code switching that is precise, consistent, transparent and thus supportive of teaching for conceptual understanding of mathematics in secondary schools.