The classical electromagnetic fields are usually described by six independent functions of space and time, namely the three components of the electric field and the three components of the magnetic field, satisfying Maxwell's equations. On the other hand it is well known that all observable effects of these fields can be derived from five components of the electromagnetic stress-energy tensor. Consequently the usual description contains one superabundance, that can be described by one parameter φ which cannot be measured experimentally but must be fixed by an arbitrary convention. Implicit conventions concerning φ, underlying the usual approach to electromagnetic fields tend to obscure the understanding and should be replaced by an explicitly stated convention. It is recommended that this convention be that φ be defined zero for electrons.