Ferroics are often characterized by the behavior of their order parameters under space inversion and time reversal (see table). The operation of space inversion reverses the direction of polarisation (so the phenomenon of polarisation is space-inversion antisymmetric) while leaving the magnetisation invariant. As a result, non-polar ferromagnets and ferroelastics are invariant under space inversion whereas polar ferroelectrics are not. The operation of time reversal, on the other hand, changes the sign of M (which is therefore time-reversal antisymmetric), while the sign of P remains invariant. Therefore, non-magnetic ferroelastics and ferroelectrics are invariant under time reversal whereas ferromagnets are not.