There are other undesirable rotor axis configurations apart from saturation, notably anti-parallel alignments. For example, if a spacecraft with two dual-gimbal CMGs gets into a state in which one rotor spin axis is facing directly forward, while the other rotor spin is facing directly aft (i.e. anti-parallel to the first), then all roll control will be lost. This happens for the same reason as for saturation; the rotors can only produce gyroscopic torques at right angles to their spin axes, and here these torques will have no fore-and-aft components and so no influence on roll. However, in this case the CMGs are not saturated at all; their angular momenta are equal and opposite, so the total stored angular momentum adds up to zero. Just as for saturation, however, and for exactly the same reasons, roll control will become increasingly difficult if the CMGs even approach anti-parallel alignment.