The possible terms appearing in the expansion above are constrained by symmetries of the material. Most notably, the tensor α α must be antisymmetric under time-reversal symmetry. Therefore, the linear magnetoelectric effect may only occur if time-reversal symmetry is explicitly broken, for instance by the explicit motion in Röntgens' example, or by an intrinsic magnetic ordering in the material. In contrast, the tensor β β may be non-vanishing in time-reversal symmetric materials.