Imagine someone who found secondary-school maths difficult being grilled about logarithms. That is how a lot of equity investors look if you ask them about currency risk. It is not because the question is novel: any client can spot that the share price of an overseas firm, or one doing business across borders, ought to depend on foreign-exchange (FX) rates. It is because it is easy to pose, but maddeningly hard to answer. Forecasting earnings is already a pain. It becomes much worse when the task is to make forecasts for each company in a portfolio, before splitting costs and revenues by perhaps a dozen currencies, and then netting it all off against hedging arrangements made years ago by a now-retired treasurer. Unsurprisingly, such analysis is often dumped in the “too hard” bucket.