This paper contributes to understanding why curriculum design in a discipline with a horizontal knowledge structure is difficult, time-consuming and contested. A previous paper on the same case study in one sociology department reported that students who had completed the general sociology major found it lacking in coherence. To illustrate the problem, I selected two third-year sociology courses, Urban Studies and Diversity Studies, and set out to compare and contrast how knowledge claims are made and legitimated in these two discourses. The paper also has a methodological focus--to demonstrate the potential of systemic functional linguistics as a method of discourse analysis that can complement and deepen a sociological analysis--Bernstein's sociology of education and in particular his concept of "grammaticality". I seek to make explicit the basis for knowledge claims in these two sub-disciplines and then to investigate how this "grammar" is built into criteria for assessing students. The long-term goal of this project is pedagogic--to understand how academic discourses work, in order to contribute to the development of more coherent curricula and visible pedagogies with explicit assessment criteria, for the enhancement of teaching and learning. The analysis shows that the "grammars" of these two academic discourses (in the same discipline, sociology) are based on different ordering principles; they are based on different ontological, epistemological and methodological assumptions. The analysis also shows that the respective "grammars" do "get into" the assessment criteria, although in a contextually contingent manner. The paper concludes by suggesting that the use of SFL as a method of discourse analysis within a social realist sociology of education framework proved to be fruitful and worthy of further development, particularly for education development work where the quest to make explicit the criteria for producing a "legitimate text" is critical. (Contains 2 notes and 3 tables.)