Abstract: We present a pen-based intelligent tutoring system (ITS) for undergraduate Statics which scaffolds students in the construction of free body diagrams and equilibrium equations. Most existing ITSs rely on traditional WIMP (Windows, Icons, Menus, Pointers) interfaces, which often require the student to select the correct answer from among a set of predefined choices. Our system, by contrast, guides students in constructing solutions from scratch, mirroring the way they ordinarily solve problems, which recent research suggests is important for effective instruction. Our system employs several new techniques for sketch understanding, including a simple-to-implement stroke merging technique, a stroke clustering technique, and a technique that uses a Hidden Markov Model to correct interpretation errors in equations. Our tutoring system was deployed in an undergraduate Statics course at our university. Attitudinal surveys indicate that the tutoring system is preferable to traditional WIMP-based systems and is an effective educational tool.