# Which one is ground?

I have an AC adapter jack that has three terminals, L(eft), M(iddle), and R(ight). If I plug in the power supply and connect the plug to the jack, these are the readings I get from the multimeter.

L(red probe) M R(black probe) = +5V
L(black probe) M R(red probe) = -5V
L(red probe) M(black probe) R = 0V
L M(red probe) R(black probe) = 0V


So is the middle terminal ground? Or is ground the the right terminal? I heard that I only need to use two of the terminals when supplying power to components. Is this true?

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