The National Minimum Curriculum (NMC) of Malta recommends the use of the country’s second language, English, for the teaching of mathematics. The aim of my study was to enhance the local medium-of-instruction debate by focusing on the use and development of a mathematics register, and distinguishing between issues relevant to second-language classrooms and ones more generally applicable Assuming a social perspective of learning, I used a grounded methodology, thus generally allowing my reflections to develop out of the data I collected. The research design consisted of lesson observations in two primary classrooms and interviews with the teachers and pupils. I concluded that the use of English in class created tensions with other NMC principles; I also noted variations in the way some mathematical words were used when compared to what I might expect as part of an English mathematics register. On the other hand, the frequency of pupils’ use of mathematical vocabulary during lessons seemed to depend on the teacher’s pedagogic approach. Also applicable to general mathematics classrooms appeared to be three conditions I identified as important for word meanings to be effectively shared with pupils: frequency of use, clarity, and significance, that is, how crucial a word appeared to be when used.