India's PM fights corruption by making 86% of India's cash worthless

India is in the middle of an extraordinary economic experiment. On 8 November, Prime Minister Narendra Modi gave only four hours' notice that virtually all the cash in the world's seventh-largest economy would be effectively worthless. The Indian government likes to use the technical term "demonetisation" to describe the move, which makes it sound rather dull. It isn't. This is the economic equivalent of "shock and awe". Mr Modi's "shock and awe" declaration meant that 1,000 and 500 rupee notes would no longer be valid. These may be the largest denomination Indian notes but they are not high value by international standards - 1,000 rupees is only £12. But together the two notes represent 86% of the currency in circulation. Think of that, at a stroke 86% of the cash in India now cannot be used. What is more, India is overwhelmingly a cash economy, with 90% of all transactions taking place that way.