The multi-faceted nature of mathematics knowledge for teaching, including pedagogical content knowledge (PCK), has been studied widely in elementary classrooms, but little research has focused on senior secondary mathematics teaching. This study utilised the Knowledge Quartet (Rowland et al., "Research in Mathematics Education," 17(2), 74-91, 2005) to analyse mathematics teaching at the senior secondary level using excerpts from a lesson on differential calculus and another on discrete probability distributions. The findings reveal that, at this level, there is a complex interplay among aspects of the Knowledge Quartet, including the impact of foundational knowledge on contingent moments. Horizon content knowledge is shown to play an important role in teaching decisions, as do perceived constraints. This has implications for future research into how teachers' horizon knowledge might be expanded and into teachers' perceptions of mathematics course constraints on the enactment and development of their mathematics knowledge for teaching.