The aim of this paper is to analyze and classify tendon-driven hand exoskeletons. In the field of robot-assisted rehabilitation, solutions for home-based hand rehabilitation and motion assistance are under continuous study with the aim of reducing swelling after surgery or trauma and maintaining a healthy hand, especially for patients who suffered a stroke. Exoskeletons also assist users in activities of daily living as well as enhance their performance in repetitive tasks. These applications led to a wide literature with a variety of design solutions that actuate different combinations of hand movements; however, it is difficult to find a common approach that properly summarizes the kinematic features of a device, in particular with the emerging solutions in the field of soft robotics. As such, we here propose a new methodology to identify actuation and underactuation strategies for tendon-driven exoskeletons, with an in-depth discussion on the biomechanics of the human hand and a new classification method according to a compact nomenclature.