Banks are in the business of interest rates. They pay interest on deposits and earn interest on loans. But what happens when the rates they pay and the rates they earn don't move in sync? This mismatch creates a specific kind of risk called Interest Rate Risk in the Banking Book, or IRRBB.

Imagine a bank gives out a 30-year mortgage at a fixed rate of 3%. That's an asset for the bank, earning a steady income. To fund that loan, it might use money from customer savings accounts, where the interest rate can change frequently. If interest rates in the broader economy suddenly rise to 5%, the bank has to pay more to its depositors to keep their money. But it's still only earning 3% on that old mortgage. Its profit margin gets squeezed.

This is the core of IRRBB. It’s the risk that changes in market interest rates will negatively affect a bank's financial condition. Managing it is crucial for maintaining profitability and long-term stability.

IRRBB affects a bank's health in two main ways, offering both a short-term and a long-term perspective on the risk.

First, let's look at the earnings perspective through something called Net Interest Income, or NII.

NII is the bread and butter of most banks. It's a key measure of profitability. When interest rates change, NII can be squeezed. If a bank has many long-term, fixed-rate loans (assets) but funds them with short-term deposits (liabilities), a sudden rise in rates means it pays more on deposits while its income from loans stays the same. The result is lower NII and reduced profits.

Now for the long-term view. This is measured by looking at the Economic Value of Equity, or EVE.

EVE is a more comprehensive measure of a bank's net worth. It’s not just about this quarter's profits; it's about the total value of the bank. Think of it like this: if you own a bond that pays 3% interest, and new bonds are suddenly being issued that pay 5%, your old bond is less attractive. Its market value drops. The same thing happens to a bank's assets.

When interest rates rise, the present value of a bank’s fixed-rate assets (like mortgages and other loans) falls. The value of its liabilities might fall too, but not necessarily by the same amount. If the value of its assets falls more than the value of its liabilities, the bank’s overall net worth—its EVE—declines.

So, NII gives a snapshot of current earnings, while EVE provides a long-term view of the bank's solvency. A bank might manage its risk to protect its NII in the short term, but this could expose its EVE to risk in the long term, or vice versa. Balancing these two perspectives is the fundamental challenge of managing IRRBB.