This class II ungrounded plug with two flat parallel prongs is pretty much standard in most of North and Central America. It is known as NEMA 1-15 and was invented in 1904 by Harvey Hubbell II. The plug has two flat 1.5 mm thick blades, measuring 15.9 – 18.3 mm in length and spaced 12.7 mm apart. Type A plugs are generally polarised and can only be inserted one way because the two blades do not have the same width. The blade connected to neutral is 7.9 mm wide and the hot blade is 6.3 mm wide. This plug is rated at 15 A.
Type A and B plugs have two flat prongs with a hole near the tip. These holes aren’t there without a reason. If you were to take apart a type A or B socket and look at the contact wipers that the prongs slide into, you would find that in some cases they have have bumps on them. These bumps fit into the holes so that the outlet can grip the plug’s prongs more firmly. This prevents the plug from slipping out of the socket due to the weight of the plug and cord. It also improves the contact between the plug and the outlet. Some sockets, however, do not have those bumps but just two spring-action blades that grip the sides of the plug pin, in which case the holes are not necessary.