Abstract: A user performs a gesture with a hand-held or wearable device capable of sensing its own orientation. Orientation data, in the form of a sequence of rotation vectors, is collected throughout the duration of the gesture. To construct a trace representing the shape of the gesture and the direction of device motion, the orientation data is processed by a robotic chain model with four or fewer degrees of freedom, simulating a set of joints moved by the user to perform the gesture (e.g., a shoulder and an elbow). To classify the gesture, a trace is compared to contents of a training database including many different users' versions of the gesture and analyzed by a learning module such as support vector machine.