In this paper, we describe the design philosophy, implementation and evaluation of InkKit, an informal design platform that uses pen input on a tablet PC to imitate the informality of a low fidelity tool. The aim is for this toolkit to provide a foundation for further research into domain specific sketch support. Designers initially hand-sketch their ideas [3, 6] because informal tools, such as pen and paper, offer the freedom to work with partly formed or ambiguous designs. The emergence of electronic pen input systems has seen a number of exploratory projects applying pen-based sketch software to the design process. Even though these projects differ, most of them use the same general framework. Thus a significant part of the implementation incorporates the same basic functionalities. Categories and Subject Descriptors H.5.2 [User Interfaces]: Input devices and strategies D.2.2 [Design Tools and Techniques] User Interfaces Keywords Sketch tools, toolkits