The aim of this paper is to establish a fundamental basis for the analysis of underactuated fingers with a general approach. A new method to obtain the force capabilities of any underactuated fingers will be presented. Force capability is defined as the ability to generate an external wrench onto a fixed object with a given set of phalanges. This method is based on the introduction of two new matrices which completely describe the relationship between the input torque of the finger actuator(s) and the contact forces on the phalanges. Using this tool, one can study the conditions under which certain phalanx forces vanish, and compare different underactuation mechanisms with a rigorous approach.