No. of Recommendations: 2
The third wire is an electrical safety ground...see links others have posted that probably explain this.

Just some notes on why you need 2 or 3 prongs.

Electrically or safety-wise the 3rd prong is not needed for things like TVs and stereos, nor are they usually needed for better picture quality or sound quality. On older appliances, for example, the third wire would be connected to the outer case when it was metal. This is so you don't kill yourself if the appliance malfunctions (internal short) and you are grounded, like standing in a puddle of water at the same time. Most appliances, like drills, sanders, kitchen stuff like blenders, etc usually have plastic cases, can't have this kind of problem, by design and only have two prongs. The same goes for most TVs and stereos, but not all.

Almost all PCs have 3 prongs because they have metal cases and metal cases on the internal power supplies. Laptops have only 2 prongs because they have a small (brick-like) transformer in a plastic case and have only DC power into them.

If you have something that comes with a 3-pronged plug you need to ground the third prong to insure safety.

Some cords have only two prongs but one blade is wider. This insures that you plug the hot side (smaller) into the correct side.

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