Much of the work on teacher knowledge suggests that content knowledge shapes pedagogical content knowledge, and in fact is primary in the relationship between the two forms of knowledge. In this paper we challenge this view, arguing from a situated perspective on knowledge and learning, that pedagogical content knowledge strongly shapes content knowledge, and in fact, may be primary in some cases. We develop this argument theoretically and illustrate it with data from two contexts: Nigeria and South Africa. The data from the Nigerian context come from scenario-based teacher knowledge interviews, which we used to explore teacher knowledge in geometry. The data from the South African context come from interactions in professional learning communities of mathematics teachers. We make two, related, arguments. First, practising teachers' knowledge is strongly influenced by their practice, so that they come to know well what they teach regularly. Second, learning new pedagogical content knowledge can precede learning new content knowledge. We argue that taking the relationships between practice and knowledge seriously may help researchers develop non-deficit understandings of teachers' knowledge and at the same time work to improve both teachers' and learners' mathematical understandings.