This work introduced a new hand prosthesis architecture able to achieve an opposite and lateral grasp by using a single actuator. An analysis of the hand prostheses, as well as the movements classically performed in daily life and at work allows us to focus our work on these two types of grasp. A lockable passive joint is moved by the user to switch between the opposite and lateral grasp. The location of the thumb was defined thanks to 3D scans in four extreme positions. The movement of the thumb is analysed to determine the location of the joints to be created. Then, a four-bar linkage was optimized to realize this motion with good force transmission. A sensitivity analysis was performed to evaluate the performance index variation based on design parameters.