According to EU law, all members must keep their budget deficit to within 3% of GDP. You’d think this was based on sophisticated modelling and forecasting, but actually Guy Abeille, then a senior Budget Ministry official, came up with the figure in less than an hour. As he explains, he needed “an easy rule that he could deploy in his discussions with ministers who kept coming into his office to demand money.” And luckily for him the number 3 was “somewhat reminiscent of the Trinity.”