The kinematics and statics of rigid systems are intimately connected. The movements prevented by mutual and external constraints are in fact strictly related to the reactive forces exerted by the constraints themselves. More particularly, it will be noted, and subsequently rigorously demonstrated, how the static matrix is the transpose of the kinematic one and vice versa, a property that will re-present itself also in other chapters devoted to elastic body systems. In every case, this property will show itself to be a consequence of the principle of virtual work, each of the two theorems implying the other.