Denim is an outgrowth of the original SILK project, a pen-based sketching tool for designing user interfaces. SILK combines many of the benefits of paper-based sketching with existing electronic prototyping tools. It allows designers to quickly sketch an interface using an electronic pad and stylus. SILK preserves the important properties of pencil and paper: a rough drawing can be produced quickly and the medium is flexible. However, unlike a paper sketch, this electronic sketch is interactive and can easily be annotated and modified using editing gestures. SILK allows the designer to extend the interactivity of the recognized widgets using storyboards.