Here is the version most people carry around in their heads: a committee meets, discusses the economy, and announces the interest rate. The rate is now that number. Banks comply, because the central bank said so.
Almost none of that is right, and the way it’s actually wrong is genuinely interesting.
The rate is not a rule. It’s a target.
When a central bank announces a rate, it is not issuing an instruction. No law compels any bank to lend at that number. The central bank has not set a price.
What it has done is announce where it intends a particular interest rate to end up — and then it goes into the market and makes that happen.
Which rate, exactly?
Not your mortgage. Not your savings account. The rate being targeted is the one banks charge each other for overnight loans.
Banks hold reserves — money parked at the central bank. Every day, the flow of payments through the economy leaves some banks with more reserves than they need and others with less. The ones with a shortfall borrow, overnight, from the ones with a surplus. That enormous, invisible, daily market has a price, and that price is the rate the central bank is aiming at.
Everything else — your mortgage, your credit card, corporate borrowing, government debt — sits on top of that, priced off it with a spread. Move the base, and the whole structure above it shifts. Not instantly, and not evenly, but it shifts.
So how do they hit the target?
By changing supply.
The central bank is the only entity that can create reserves. It has, in effect, an infinite supply of the thing banks are lending each other.
Want the rate to fall? Create more reserves and put them into the system — buy assets from banks, pay for them with newly created reserves. Reserves become abundant, and the price of borrowing them drops.
Want the rate to rise? Do the reverse. Drain reserves, make them scarcer, and the price of borrowing them climbs.
The modern version is more elegant. Rather than fine-tuning the quantity, most central banks now simply pay interest on reserves. If a bank can earn 4% risk-free by leaving money at the central bank, it will not lend to another bank for 3%. Why take on risk for less than you can get for nothing?
That single fact sets a floor under the whole market. The central bank doesn’t need to enforce anything. It just makes the alternative unattractive.
Why it’s more fragile than it looks
Here’s the part that gets left out.
This entire structure rests on the assumption that the machinery beneath it keeps working — that banks keep lending to each other, that the plumbing of the payment system holds, that the market the central bank is steering actually exists.
In a crisis, that assumption can fail. Banks stop lending to each other because they no longer trust that they’ll be repaid. The overnight market seizes. And at that moment, the central bank’s announced rate becomes a number with nothing underneath it — a target it is aiming at a market that has stopped functioning.
This is why central banks respond to financial crises with far blunter tools than rate changes. When the transmission mechanism breaks, moving the rate accomplishes nothing. You have to fix the plumbing first.
The short version
- The central bank doesn’t set your interest rate. It targets the rate banks charge each other overnight.
- It hits that target by controlling the supply of reserves, and by paying interest on them.
- Everything else is priced off that base, with a spread.
- The whole system depends on the underlying market continuing to function — and in a crisis, it may not.
It’s less like a dial and more like a very large lever, connected to the economy by a long chain of assumptions, most of which usually hold.
