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A standard 15-amp 120 volt socket (NEMA 5-15) will have two vertical parallel slots like this (| |) and also have a third round hole for the ground.
The wider slot connects to the white neutral conductor (grounded conductor) and the narrow slot connects to the black live (or hot) conductor.
You must make sure to preserve polarity when replacing these.
A 15-amp 240 volt socket (NEMA 6-15) will have two slots in tandem like this: - - plus a third round hole for the equipment ground conductor.
The 20 amp version (NEMA 6-20) will have one vertical and one horizontal (| -).
The colors of these wires (in the ones I've opened) are usually black and red. The green is always supposed to be used for GROUND.
Now here is where it gets confusing: The 120 volt/20 amp socket (NEMA 5-20) looks like this: (- |) - a mirror image of the 240 volt 20-amp socket.
A 120 volt socket must NEVER be used on a 240 volt circuit for just that reason - CONFUSION by an end user.
You might want to test all the outlets in your house and have an electrician replace whatever looks bogus (if you don't know how to do it).
Joe, feel free to correct me.