The most important number in finance on its way out: All you need to know

In the world of finance, there is one number that arguably matters more than any other. You can find it in the small print on adjustable-rate mortgages and private student loans, it is the basis for enormous corporate loans, and it underpins nearly $200 trillion of derivatives contracts.

But it is on the way out, and Wall Street has not worked out how to replace it. The number in question is called Libor, which is short for the London interbank offered rate. Published daily, Libor is an interest rate benchmark, or the basis for many other interest rates. If you have