As I settle down on the back seat of the bus, I watch as my brother-in-law confidently gets on board and touches his Suica card to the scanner.

Beep beep. It¡¯s rejected ¡ª lack of funds.

Not to worry. While he doesn¡¯t speak the local language, this seasoned traveler isn¡¯t fazed. He readies some cash instead. But a minor kerfuffle begins. I head to the front and ask the driver what the issue is. Looking relieved to hear some Japanese, he explains that the shiny new ?500 coin that my brother-in-law has paid with is simply too new for his bus. It can¡¯t be accepted by the machine.

The issue is small. I offer up a ?1,000 note, and the matter is settled.

But it got me wondering. When I used to ride the bus as a child around my native Yorkshire, we paid cash. And the cash was handled by the driver, who had a manual change-giving machine and a bag for the overflow.

But assuming that the system in Japan is now set up so a driver never needs to personally handle any cash, it raises the question of why certain items of legal tender can not be used on public transport ¡ª or indeed other places.

A quick search online shows that nonacceptance of the new ?500 coins ¡ª the silver and gold colored ones issued since late 2021 ¡ª definitely occurs elsewhere. A Kyoto Shimbun article from last year reports the same problem with vending machines and parking meters in the prefecture, for example.

So what¡¯s the problem? Is it the weight? The design? The dimensions?

The Bank of Japan¡¯s website tells us that the new coin weighs 7.1 grams, while the old coin is 7. They are exactly the same diameter, at 26.5 millimeters, and both have ridges on the edge, though these are slightly different on the new coin.

They definitely look different, with the new one appearing rather similar to a British ?2 or even bearing a vague resemblance to a €2 coin. But machines surely pay no attention to looks. So it seems to be all about that crucial 0.1 gram difference and those ridges.

In fact, as well as commemorative editions, there are three versions of the ?500 coin that are legal tender in Japan. The first fell out of favor after it was deemed too easy to counterfeit or imitate, partly due to its similarity to the Korean 500 won coin of the early 1980s.

The other two have partly been the product of efforts to clamp down on counterfeiting, with each having fancier and more impressive security features than the last.

But the number of new...