The moment of a force is only defined with respect to a certain point P (it is said to be the "moment about P ") and, in general, when P is changed, the moment changes. However, the moment (torque) of a couple is independent of the reference point P : Any point will give the same moment. In other words, a couple, unlike any more general moments, is a "free vector". (This fact is called Varignon 's Second Moment Theorem.)