Frequently writing is now no longer the central mode of representation in learning materials--textbooks, Web-based resources, teacher-produced materials. Still (as well as moving) images are increasingly prominent as carriers of meaning. Uses and forms of writing have undergone profound changes over the last decades, which calls for a social, pedagogical, and semiotic explanation. Two trends mark that history. The digital media, rather than the (text) book, are more and more the site of appearance and distribution of learning resources, and writing is being displaced by image as the central mode for representation. This poses sharp questions about present and future roles and forms of writing. For text, "design" and "principles of composition" move into the foreground. Here we sketch a social semiotic account that aims to elucidate such principles and permits consideration of their epistemological as well as social/pedagogic significance. Linking representation with social factors, we put forward terms to explore two issues: the principles underlying the design of multimodal ensembles and the potential epistemological and pedagogic effects of multimodal designs. Our investigation is set within a research project with a corpus of learning resources for secondary school in Science, Mathematics, and English from the 1930s, the 1980s, and from the first decade of the 21st century, as well as digitally represented and online learning resources from the year 2000 onward. (Contains 9 figures.)