This paper deals with the theoretical background of magnetic reconnection including systems where the magnetic field does not vanish (general magnetic reconnection). The magnetic field is represented by Euler potentials. The time evolution of the Euler potentials as seen by the moving plasma provides an appropriate formulation of the properties of general magnetic reconnection. A critical review of the concept of magnetic merging is given, and it is shown that in certain situations there may be ideal plasma flow across a separatrix which does not involve nonideal effects. It is shown that such problems do not arise within the concept of general magnetic reconnection. It is demonstrated that general magnetic reconnection with nonvanishing magnetic field crucially depends on properties of the parallel electric field.