Teachers' pedagogical design capacity is their ability to perceive and mobilise existing resources to create productive instructional episodes in the classroom. To a certain extent, this ability is dependent on the curricular resource used. As the textbook remains the most commonly used curricular resource in mathematics classrooms, the study reported in this paper investigates how and why one experienced mathematics teacher utilises the textbook. Data were gathered using lesson observations, as well as pre-lesson and post-lesson interviews. The teacher used offloading on the textbook, adapting the textbook content and improvising in the lessons to varying degrees, being aware of the affordances and constraints that the textbook has for her teaching practice. That approach to the textbook enabled the teacher to create various opportunities that enhance learning. The results of the study indicate that the mathematics teacher's awareness of what a particular resource offers for teaching practice, and what constraints could be encountered on this journey are significant in terms of the teacher's design capacity.