Recent research has called for more attention in the understanding and developing of mathematics teaching talk, focusing on the specific use of language as a resource to help students learn mathematical concepts. Based on a task-based interview with eleven teachers, this paper attempts to utilise the Mathematics Register Knowledge Quartet to examine what these teachers know and may do when teaching fractions. Consequently, the analysis of the teachers' responses, in relation to the four dimensions of the quartet, provided insights on how teachers might attend to or use (or not) the mathematics register in the mathematics teaching talk of fractions and related concepts.