To explore the role of the textbook as a context variable in process—product relationships, data on teaching practices and learning outcomes from the IEA Classroom Environment Study in The Netherlands were used. The sample consisted of 50 secondary school mathematics classes and their teachers. Three textbooks were represented in the sample. Data collection included systematic observation of lessons and administration of tests and questionnaires to students and teachers. Nine teaching practices analogous to the distinguishing features of the textbooks were identified, and data relating to these practices were analysed. Four of the teaching practices occurred to different degrees in the three groups of textbook users. The correlations with two learning outcomes in different groups of textbook users were significantly different for 5 of the 18 cases. These results suggest that the textbook is an important context variable.