Premodern manuscript production was fluid. Books and papers freely changed hands, often against their authors’ wishes. In the absence of copyright laws, certain countermeasures arose. This study considers one of them: self-commentary, meaning an author’s explanations on his own works. The article deals with two cases of medieval self-commentary across linguistic and cultural boundaries: the Arabic author and rationalist Abū al-ʿAlāʾ al-Maʿarrī (d. 1057 CE), and the professional Byzantine littérateur John Tzetzes (d. 1180 CE). After an overview of their lives and works, with a focus on the key role of self-explanation, the article considers their respective manuscript cultures, which involved face-to-face educational settings that nonetheless permitted widespread copying. There follows a discussion of textual materiality, which reveals a mutual concern to avoid tampering or misinterpretation. Then, the article shows how both men tried to direct readers by exploiting language’s capacity for multiple meanings. The conclusion ponders the relevance of this study for problems posed by digital book technology.