I was surprised to learn this. My Brazilian friend said you just had to know what outlets in the house where 110 and which were 220. If you got it wrong then the magic smoke came out.
Nothing wrong with the socket itself but the use for different voltages is terrifying.
Not pictured, but the US also has a socket that looks like type B, but it is for 220-240v (mostly used for window air conditioners, or tools). It is not compatible with the normal type A/B socket/plugs though because one of the pins is sideways.
I once saw a house in the US which had outlets that would accept either type, and you had to know if it was 120V or 240V. IIRC this was a type A socket, but it would accept either voltage.
I think you're confusing a NEMA 5-20 (120V, 20A) and NEMA 6-20 (240V, 20A). Each has one blade sideways but they're mirror images of each other. T-slot varieties of both outlets are common, which can accept both 15A and 20A versions of their respective voltages. You won't find outlets that can accept both a 120V and 240V plug.
This house had sockets that would accept NMEA 5-15, 5-20, 6-15 or 6-20. (I forgot the numbers until you mentioned them). It wouldn't be allowed now, but my guess is the house was built in the 1950s.
Those outlets existed because pre-NEMA there were competing parallel and tandem bladed plugs/sockets [1]. The plugs you saw were probably like examples 2 & 3 in that link. The parallel configuration became NEMA 5-15P and the tandem became NEMA 2-15P.
The NEMA 6 series is wider than the 5 series and the 6-15P probably won't fit in those old outlets, but the NEMA 2-15P would. . . if you could find one.
Ah yes, I have seen those before (quite rare). IIRC they don't have holes for the grounding prong, which should prevent plugging in any modern 240V appliance.
I should expect that they ought to mark each one with the voltage label to know what it is, if they do something like that.
(Using a different shape might be even better, to avoid plugging it wrong by accident or if it is dark and the label cannot be read or it is faded and cannot be read, etc, and you can more easily tell which device works with which plug because the shape matches the devices being plugged too.)
Multiple AC standards is just fine, so long as all the sockets are physically different so you can't screw up. Electricians are expected to know all the weird rules and get things right, but normal people can't make a mistake so it works.
Almost all US houses have two voltages, but since the sockets are obviously incompatible it all works. I've seen industrial buildings that have more than one grid connection (and in one case they paid extra to ensure the grid connections went to different sub stations)
I was surprised to learn this. My Brazilian friend said you just had to know what outlets in the house where 110 and which were 220. If you got it wrong then the magic smoke came out.
Nothing wrong with the socket itself but the use for different voltages is terrifying.