The other day I was paying for my groceries at the supermarket when a man rushed up to the check-out clerk and asked her if she would change a HK$1,000 banknote. Apparently he had tried to offer this thing to a taxi driver and had quite rightly been asked if he had any real money, which he didn't, so he ran into the nearest supermarket and tried to exchange it there. The check-out clerk was having none of it. Only recently the tills in the shop had borne a notice warning that HK$1,000 banknotes of certain years would not be honoured as there had been too many counterfeits. But she had the perfect excuse anyway. He wasn't a regular customer, so tough luck. I took pity on him, gave him real money for it, and then had to puzzle where I could lay it off and whether it was genuine. Thus, here is a little surprise for you. If you took all the HK$1,000 banknotes presently in circulation and laid them end to end you would have a line extending more than three quarters of the way round the world. At the end of 2013, when last tallied by the Hong Kong Monetary Authority, there were 181.7 million genuine examples of these putrid yellow pieces of paper floating around somewhere, and who knows how many fakes. They accounted for 58 per cent by value of all banknotes in issue. Yes, that's right. By value, more than half of the banknote issue consists of banknotes that you don't want to carry because so many merchants won't accept them. We all swear when we push the wrong button and the ATM spits one out. They are only used to any extent in the Jockey Club's betting outlets. What is more, as the first chart shows, the proportion of these things in currency in circulation has steadily risen over the years. People take less of them in bad times, more in good times, but the overall trend is up. And here is another factoid for you. Despite the growth of more sophisticated payment systems we still rely - increasingly - on cash. Currency in circulation has risen from less than 6 per cent of gross domestic product in mid-1997 to 14.5 per cent as of the latest numbers. Notice from the second chart that the same ratio in Britain is much lower than it is here and has risen nowhere near as rapidly in recent years. What are we to make of all this? I have a simple answer. The HK$1,000 banknote is the money launderer's medium of choice in this town and its popularity is rising (about six times more of them in circulation than in 1997) because so much of our commercial relations with the mainland must still remain hidden from Beijing. That is why we need so much cash in big denominations. Bear this in mind the next time you hear the monetary authority making moral noises about the evils of money laundering. They are aware of it. They condone it.