Grouping students by ability into courses with distinct curricula, or "tracking" as it is called in middle and high schools, has provoked a furious debate among educational scholars and practitioners. Research on tracking invariably characterizes the practice as a school-level, unitary phenomenon; schools are depicted as either tracked or untracked. One consequence of such thinking has been to narrow educators' policy options to either acceptance or rejection of tracking. By examining the policies of 373 California middle schools, this study challenges the characterization of school curricular policy as a unitary construct and presents evidence that tracking, as practiced in schools, evolves from its interaction with subject matter. The movement to reduce ability grouping, for instance, has achieved greater success in English courses than in mathematics. When viewed as a product of subject area subsystems, tracking's place in school organization comes into sharper focus. Data were gathered through: (1) a survey of all California middle schools, which elicited an approximate 42 percent response rate; and (2) case studies of 23 California middle schools, which included interviews with 175 principals and teachers. One table and one figure are included. The appendix contains case-study statistics. (Contains 39 references.) (LMI)