We determine the different symmetry classes of bi-dimensional flexoelectric tensors. Using the harmonic decomposition method, we show that there are six symmetry classes. We also provide the matrix representations of the flexoelectric tensor and of the complete flexoelectric law, for each symmetry class.Symmetry 2020, 12, 674 2 of 29 concern this aspect of the project. Furthermore, this task has to be undertaken by considering the final application, hence by trying to give a physical content to invariants.Piezoelectric and flexoelectric effects can be introduced via the following constitutive equation [1,5]:where p is the polarization, q the electric field, ε ∼ the strain tensor, and η the strain-gradient tensor.The second-order tensor S ∼ , the third-order tensor P , and the fourth-order tensor F ≈ describe the dielectric effect, the direct piezoelectric and flexoelectric effects [5], respectively. It is important to note that in centrosymmetric materials, the third-order tensor P vanishes. This means that piezoelectricity exists only in non-centrosymmetric materials.The vector space of flexoelectric tensors is denoted by Flex. For the optimal design of flexoelectric structures, the understanding of the algebraic structure of Flex is mandatory. Important issues concerning Flex include the following questions: