It's unlike grounded outlets, where both slots are wide open and exposed all the time. In a TRR, you can't insert an object like a key, hairpin, or fork into just one of the openings.

I use power strips and multi-plug extension cords to power my outdoor smart lights, water feature pumps, barbecue, and more. This box keeps everything hidden and dry. By Michael Brown ...

Conversely, the British Type G plug was invented in 1947 with British Standard 1363, in an effort to create safer universal outlets for new postwar housing.