Sorry for such a basic question, but I'm having trouble grasping the idea of why the Thevenin equivalent of a network (as seen from two terminals) almost always has a Norton equivalent in the form in which it has been defined. More specifically, I've been trying to work out from staring at the diagram why an ideal voltage source in series with a resistor is equivalent to an ideal current source in parallel with a resistor. (why specifically in parallel in the the latter case and in series in the former?) Everytime I stare at it, I think I'm close to an answer, but I can't put it into words resoundingly...to make a statment like..."the current source has to be in parallel with the resistor, NOT in series or in any other configuration, in order for blah blah blah such and such to be the same as it is in the Thevenin version!" Any help would be appreciated.