It has long been believed that when it comes to interest rates, zero is as low as you can go. Who would choose to keep their money in the bank if they had to pay for the privilege?

But for the people who control the world's money, this idea has recently been thrown out of the window. Many central banks have pushed their rates into negative territory and yet the financial system has still to come to an abrupt end.

It is a discovery that flips on its head the conventional idea of how authorities could respond to future economic crises; and for central bankers, this has come as a relief.

Central bank policymakers had believed they had run out of room to support their respective economies, with their interest rates held close to the floor.

Traditionally, it was thought that if you wanted to boost the economy, the central bank would reduce its interest rates. Normally, the rates offered on savings accounts would follow, and people would choose to spend more, and save less.

But there's a limit, what economists called the "zero lower bound". Cut rates too deeply, and savers would end up facing negative returns. In that case, this could encourage people to take their savings out of the bank and hoard them in cash. This could slow, rather than boost, the economy.