Why do some receptacle outlets have a T-shaped hole?

Why do some receptacle outlets have a T-shaped hole? t slot receptacle However, the 20A 5-20R receptacle will accept both a 5-15 plug and a 5-20 plug Those are the ones with the T slot This is why there are 15 the wild hatter slot You don't see it often, but non-polarized North American outlets exist, where both slots are the same size It was more common decades ago,

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