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We analyze static grasp stability of a single planar object. In our previous papers, we considered that each finger is replaced with an orthogonal virtual spring model. This means that each finger is constructed with prismatic joints. Human hands and the most developing mechanical hands are constructed with angular joints. In this paper, we investigate the effect of fingertip rotation and angular joints. The stability is analyzed from the potential energy method. A grasp stiffness matrix is derived by considering not only frictional rolling contact but also frictionless sliding contact. The gravity effect of the object is also considered. The difference between the frictional stiffness matrix and the frictionless one is also obtained. The stability is evaluated by the eigenvalues and eigenvectors of the grasp stiffness matrix. Effectiveness of our method is demonstrated through numerical examples.