The paper begins by arguing that knowledge-blindness in educational research represents a serious obstacle to understanding knowledge-building. It then offers sociological concepts from Legitimation Code Theory--"semantic gravity" and "semantic density"--that systematically conceptualize one set of organizing principles underlying knowledge practices. Brought together as "semantic profiles", these allow changes in the context-dependence and condensation of meaning of knowledge practices to be traced over time. These concepts are used to analyze passages of classroom practice from secondary school lessons in Biology and History. The analysis suggests that "semantic waves", where knowledge is transformed between relatively decontextualized, condensed meanings and context-dependent, simplified meanings, offer a means of enabling cumulative classroom practice. How these concepts are being widely used to explore organizing principles of diverse practices in education and beyond is discussed, revealing the widespread, complex and suggestive nature of "semantic waves" and their implications for cumulative knowledge-building. (Contains 9 figures.)