Type A is the flat two-blade plug that powers most of North and Central America and Japan, and it carries no earth at all. The design is about as spare as a plug can be: two flat parallel pins, set on 12.7 millimetre centres, often with a small round hole punched near the tip of each blade. Those holes are not decoration. In older sockets, spring-loaded bumps inside press into them and grip the blades, holding the plug in place and giving the contacts a firmer bite. Over time the American version picked up a safety refinement.

To keep people from inserting a plug the wrong way round, the neutral blade was made wider than the live one, so a polarized Type A will only enter a matching socket in a single orientation. The plug learned to have a right way up. Japan took the same basic shape but never adopted that widening, keeping both blades the same size. This small difference produces a famously lopsided compatibility. Because Japanese blades match the original American dimensions, a Japanese plug drops happily into an American socket.

But an American polarized plug, with its one fat neutral blade, often will not fit a Japanese socket, whose slots are cut for two equal pins and simply refuse the oversized one. The traffic runs easily in one direction and jams in the other. It is a neat illustration of how a standard drifts once it crosses a border. Same pins, same spacing, same unearthed simplicity, and yet a single decision about the width of one blade decided which travellers could plug in where. Type A is the most basic plug in wide use, and even it hides a quiet asymmetry between two countries that share its shape.