To characterize the uniaxial deformation of a tension bar, the kinematic quantities displacement u andstrainε =du/dx havebeen introduced (Section 1.2). We will now explore how the deformation of a plane or spatial structure can be described. Here, we first restrict our attention to the deformation in a plane. To this end we consider a disk where two squares { and | are marked such that they are tilted against each other (Fig. 3.1).