We explore some key constructs and research themes initiated by Jim Kaput, and attempt to illuminate them further with reference to our own research. These "design principles" focus on the evolution of digital representations since the early 1990s, and we attempt to take forward our collective understanding of the cognitive and cultural affordances they offer. There are two main organising ideas for the paper. The first centres around Kaput's notion of outsourcing of processing power, and explores the implications of this for mathematical learning. We argue that a key component for design is to create visible, transparent views of outsourcing, a transparency without which there may be as many pitfalls as opportunities for mathematical learning. The second organising idea is Kaput's notion of communication and the importance of designing for communication in ways that recognise the mutual influence of tools for communication and for mathematical expression.