To define a system of electromagnetic units uniquely, six basic quantities must be chosen. A suitable choice may include one basic unit and five constants, for example, or four basic units and two constants. Maxwell's equations are derived in a general form, independent of any particular choice of the six basic quantities. lt is shown that the MKSA-formulation of the field equations involves three different unit systems. In the relation B = u0 H, the symbols B and H are called a Giorgi-pair; other Giorgi-pairs are shown to exist in the MKSA-formulation of electromagnetic theory. Although of different dimensions, the members of a Giorgi-pair always describe the sarne physical phenomenon, and may thus be used arbitrarily, provided the Haines rule is observed. The general form of Maxwell's equations is derived for material bodies. lt is shown that the MKSA-formulation uses two conflicting models for polarizable materials, leading to two electric and two magnetic fields, and the întroduction of two units for each field in order to remove the constants from Maxwell's equations. Dimensional equations are replaced by more general expressions, called logometric formulas, including ail six basic units and constants; they are used to analyze the existing unit systems.