Why do some plugs have one wider prong than the other normal one while others have two normal prongs with the same size?

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What I’m talking about is, for example, a keurig coffee machine having two differently shaped prongs where one prong is bigger and wider than the other, so you have to insert the wider prong into the wider port.

However, something like a phone adapter doesn’t have this, and the two prongs are the same size and shape.

Why is this?

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