A study was conducted with a high school teacher and her 58 Algebra 1 students, with the purpose of gaining insight into students' difficulties with writing equations of parallel and perpendicular lines. Chevallard's theory of didactic transposition was employed in order to account for the relativity of the mathematical knowledge with respect to the institutions where the knowledge was created. During the process of didactic transposition, the mathematical knowledge lost its essential feature, the proof, with dramatic consequences for the school algebra curriculum. What remained to be taught and learned was how to execute tasks. As predicted by mathematicians, this utilitarian view of the curriculum affected the actual process of teaching and learning by focusing on executing basic tasks, and resulted in teachers and students having difficulty executing those tasks.