

It’s the oldest standard on the list, and is still around older homes in the USA. They used to be equally-sized (aka unpolarized), but later on they had a bigger hole for neutral (polarized). They’re not up to code anywhere anymore though, you’ll only see them in older houses before the grounding pin was required.
With one of my dogs, I’ll look for the dog that does exactly what I say when I say it. For the other, I will look for the dog that completely ignores everything I say.