This case study examines the differences between mathematical and everyday language, the need for both kinds of speech in the classroom, as well as some of the problems that can occur. Lesson observations, written work and interviews were conducted with the students and teachers of a Year 9 middle-attaining set in a UK comprehensive school. This data is used to explore how teachers switch between registers, the benefits and drawbacks of different translations between registers and ambiguity created by the different meanings of the word “regular”. The study concludes that, while two registers are in play, the boundaries between mathematical and everyday speech are often blurred. This creates a specific set of problems around how to interpret speech in the classroom and how to encourage students’ use of mathematical register.