The prototypical example is the formation of the non-centrosymmetric magnetic spiral state, accompanied by a small ferroelectric polarization, below 28K in TbMnO 3. In this case the polarization is small, 10 μC/cm, because the mechanism coupling the non-centrosymmetric spin structure to the crystal lattice is the weak spin-orbit coupling. Larger polarizations occur when the non-centrosymmetric magnetic ordering is caused by the stronger superexchange interaction, such as in orthorhombic HoMnO 3 and related materials. In both cases the magnetoelectric coupling is strong because the ferroelectricity is directly caused by the magnetic order.