This paper extends a dialogue on the nature of explanations and instructional discourse to include instructional explanations. The paper examines instructional explanations at three levels: (a) the distinctions between specific types of explanations (common, disciplinary, self, and instructional) with respect to specific features (problem type, initiation, evidence, form, and audience); (b) the occasions within history (events, structures, themes, and metasystems) and mathematics (operations, entities, principles, and metasystems) that prompt explanations; and (c) critical goal states present in successful explanation (representations known, verbal discourse complete, nature of problem understood, principles accessed). Using these three levels, three examples of shared instructional explanations are explored, two in history and one in mathematics.