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It has a center contact, which matches up to the very tip of the screw-in lamp, and the outer shell which actually has the threads in the shell. Now we have to discuss the significance of connecting the proper conductor to the screw shell. We have to connect the grounded conductor to the shell. If we hook this up backwards, then when a person screws in a lamp and accidentally touches the metal part of a lamp, they would receive a shock. It is a seemingly minor miswiring like this that can severely shock or even electrocute someone. That is why the Code is so specific about identification of conductors and terminals. As a final reminder, the last paragraph in Article 200 is 200.11, Polarity of Connections, which is just one sentence that states No grounded conductor shall be attached to any terminal or lead so as to reverse the designated polarity. As you can see from the screw shell example above, a simple mistake that would result in a reversed polarity could result in injury or worse.
