Several studies of disciplinary didactics (Norw. fagdidaktikk) describe the field as expansive and changing. The article interprets perceptions of such dynamic changes as results of encounters between given knowledge and new contexts in which disciplines and school subjects communicate, such as practices and projects, and hence as products and processes. It is argued that the expansion will challenge the so-called disciplinary core and that growth in turn generates both general and comparative disciplinary didactics. Definitions of disciplines and school subjects have traditionally tried to trace and segregate significant core elements. However, studies of ‘core disciplinary’ in comparative perspective rather seem to conceive similarities and differences more in terms of accentuation of certain shared/not shared elements or aspects. A key hypothesis is that this emphasis is influenced by how disciplines are related to certain constitutive aspects of (communicative) utterances. This means that focus on communicational aspects such as form, content, action, time and place will correspond with prioritised disciplinary aspects such as aesthetics, epistemology, ethics, chronology and topology, respectively. It is further described how disciplines stress their ‘essential’ disciplinarity by emphasising certain main and sub-aspects in such ‘sets’. It is concluded tentatively that theories of communication can help revealing systemic connections in the didactic field between developmental patterns of key concepts such as knowledge, context, comparison and communication.