These systems are subjected to nonholonomic constraints, and at least one degree of freedom is not actuated. This class of systems is well exemplified by robotic skateboards and snakeboards. The main difference between these systems is that the front and rear wheels of a skateboard are coupled in orientation (which ends up in only one actuated degree of freedom) whereas they are actuated independently in orientation in the case of a snakeboard (which ends up in two actuated degrees of freedom). The nonholonomic nature of these systems results from the fact that they are platforms mounted on wheels, thus subjected to roll-without-slip constraints. As for the underactuation, it results from the non-actuation of the wheels in traction, by contrast with car-like vehicles. The combination of nonholonomy and underactuation renders the control of these systems particularly challenging.