Every few weeks a committee of central bankers meets, debates, and announces a single number: the interest rate. Markets hang on the decision because that number ripples through the entire economy. Setting it is the heart of monetary policy — the central bank's management of interest rates and the money supply to keep prices stable and output steady.
The central bank has a toolkit. Its workhorse is open market operations, the buying and selling of government bonds. To loosen policy, the bank buys bonds, which pumps money into the banking system and pushes interest rates down. To tighten, it sells bonds, draining money and pushing rates up. It can also change reserve requirements, the fraction of deposits banks must hold, which alters how much they can lend.
All of this is aimed at one target. The policy interest rate is the short-term rate the central bank steers toward. It is the anchor for almost every other rate in the economy, from mortgages to business loans.
Why does a single short-term rate matter so much? Because of monetary policy transmission — the chain that carries the central bank's decision out into the real world. A lower policy rate cuts borrowing costs. Cheaper borrowing encourages firms to invest and households to buy homes and cars. That extra spending lifts aggregate demand, which raises output and, eventually, prices. The chain runs in reverse when the bank tightens.
There is a subtlety borrowers must respect. The nominal interest rate is the sticker rate the bank quotes. The real interest rate subtracts expected inflation from it, revealing the true cost of borrowing in purchasing power. If you borrow at 5 percent while inflation runs at 3 percent, your real cost is only 2 percent. Decisions that matter respond to the real rate, not the nominal one.
Monetary policy has a hard floor, and 2008 exposed it. The zero lower bound is the problem that a central bank cannot easily cut its policy rate much below zero, because people would rather hold cash than pay to keep money in the bank. When rates hit the floor and the economy still needs help, the usual lever stops working.
Central banks responded with an unconventional tool. Quantitative easing (QE) is the large-scale purchase of long-term bonds and other assets, designed to lower long-term rates and inject money even after short-term rates have bottomed out. It became the defining policy of the decade after the global financial crisis.