According to the principles of mechanics, having acquired the concept of “quantity”, if we wish to define the sum of two homogeneous quantities it is necessary to introduce the concept of “magnitude” as the ratio between the quantity under consideration and another one, homogeneous to it, chosen as unity. We define a “dimensional system” as the whole set of derived quantities whose units can be obtained from certain predetermined “fundamental” units. Among the various dimensional systems we mention those of: geometry, where all the quantities can be derived from one fundamental quantity represented by the length L; kinematics, where all the quantities can be derived from those of length L and time T; statics, where all the quantities can be derived from those of length L and force P, or (see 1.4) length L and specific force; mechanics in general, in which all the quantities can be derived from the above three fundamental ones.