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 green wire is the ground wire, the white wire is the neutral wire, and the black wire is the hot wire. Light-duty interior extension cords often lack the ground wire, but if a ground wire is present, be sure to connect it.

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.