The third prong on an electrical cord is the ground plug. Since plenty of cords have only two, it may not seem terribly important—but according to national electric code, it's an important safety feature that could save your life. Literally.

The third prong in the extension cord provides a path to the ground wire in a household electrical circuit. This ground wire greatly reduces the risk of electrical shock and fires. The three-prong cord itself should only be used with properly grounded three-slot outlets.

The first prong goes into the neutral slot in a wall power outlet, while the second goes into the hot slot. Consequently, non-grounded plug cords have an increased risk of causing electrical shock and damage to appliances due to the lack of a grounding mechanism to function as a circuit breaker.

Do not “daisy chain,” that is, plug surge protectors into extension cords or other surge protectors. This practice voids the NRTL listing and the manufacturer's warranty.