Type A vs. Type O

Type A is the ungrounded 2-flat-pin plug standard in North America, Central America, and Japan -- NEMA 1-15, rated 15A/125V. It's a physical subset of Type B: any Type A plug fits directly into a Type B socket, the ground pin recess simply unused.

Type O is Thailand's standard, introduced in 2006 -- three round pins whose earth-pin offset happens to numerically match Type B's, letting Thailand's real-world hybrid sockets also accept American Type A/B and European Type C plugs. Pure single-standard Type O outlets barely exist in practice.