This paper examines the technological affordances of the visualiser, and what teachers actually do with it in the primary (elementary) classroom, followed by an investigation into one example of teaching and learning with this whole-class technology. A visualiser is a digital display device. Connected to a data projector, whatever is in view of the camera is magnified in real time on the screen--be it a page from a book, an object, a living thing, or actions on and gestures around it. Used as a site for interaction with the class, a site for observation, a site for information, a site for modelling, a site for guided investigation and a site for assessment, scale directs attention to the screen display, whilst disconnection of the user's body compels students to make decisions about where to look when. Modelling (or demonstration) is a common pedagogic practice. Showing a class of 9- and 10-year-olds how to transform numerical values into a bar graph on the visualiser entailed multimodal ensembles of inscription, gesture and speech. Students' subsequent production of bar graphs was framed by imperatives and choices, to which they responded agentively. The ways in which teachers use the visualiser in their classroom practices has implications for mediation and communication in the classroom, and hence for pedagogy and learning. (Contains 5 figures.)