We assert that the product of user interface design should be not only the interface itself but also a rationale for why the interface is the way it is. We describe a representation for design based around a semi-formal notation which allows us explicitly to represent alternative design options and reasons for choosing among them. We illustrate the approach with examples from an analysis of scrolling mechanisms. We discuss the roles we expect such a representation to play in improving the coherence of designs and in communicating reasons for choices to others, whether designers, maintainers, collaborators or end users.