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

converting 20 amp outlet to 15 amp t slot receptacle Choose from our selection of outlet strip T slots in a wide range of styles and sizes In stock and ready to ship the great egypt slot 5-20R T-slot receptacle mounted with the hole for the ground pin at the top The neutral connection is the wider T-shaped slot on the right All NEMA 5

theater slot zeist outlet is to look at the prong style A 15A outlet has 2 vertical slots, while a 20A outlet has a third slot that resembles a sideways T X Research