Yeah, in an ideal world we would all use "common" for the one concept and "ground" or "earth ground" for the other. But in practice we use "ground" to mean "com

Yeah, in an ideal world we would all use "common" for the one concept and "ground" or "earth ground" for the other. But in practice we use "ground" to mean "common" and "earth ground" to mean "earth ground". I remember, when I was a beginner, being completely confused about what all those ground symbols meant. That was one of my first big stumbling blocks in electronics. The answer is that they're usually nothing but notational: They save you drawing some wires (every ground symbol is assumed to be attached directly to every other ground symbol by a very-low-resistance wire [1]) and more importantly they define your reference voltage (once you've drawn a ground, you can say things like "this point in the circuit is at 3.2 volts", instead of "this point in the circuit is at 3.2 volts relative to Point X".) --- [1] There are times when the connections from ground to ground have too high a resistance, or when you pour so much current into ground that the ground-lead resistance can't be ignored anymore. Those are called ground loops, and are topics for advanced class. I have a nasty ground loop when I connect my stereo system to my cable TV. It manifests as the sixty-hertz hum from hell. http://en.wikipedia.org/wiki/Ground_loop_(electricity) http://en.wikipedia.org/wiki/Ground_loop_(electricity)