A price the central bank cannot simply command
The previous guide left you with a target. A modern central bank decides what it wants short-term interest rates to be — its [[policy-interest-rate|policy interest rate]] — and the whole economy then watches that one number. But pause on a puzzle the headlines skate over. When the announcement says the bank "set rates at 4.5%," no law has been passed forcing your bank to lend at 4.5%. An interest rate is just a *price* — the price of borrowing money — and prices are set by supply and demand, not by decree. The central bank cannot abolish that fact. So how does it make the market price land exactly where it wants?
The answer is the secret of central banking: it does not *command* the price, it *engineers* it. The bank acts on one very specific market — the market where commercial banks lend their spare reserves to each other overnight — and by changing the supply of those reserves, it pushes that one rate to its target. Everything else, from your mortgage to corporate bonds, then floats on top of that anchor. The tools in this guide are simply the different ways the central bank reaches into the supply of reserves to move that single, foundational price.
The main tool: open-market operations
The workhorse, by a wide margin, is [[open-market-operations|open-market operations]] — the central bank buying and selling government bonds in the open financial market. The trick is to follow what the bank pays *with*. When the central bank buys a bond from a commercial bank, it does not pay with money it earned; it simply creates fresh reserves and credits them to that bank's account. Bonds flow in to the central bank, brand-new reserves flow out into the banking system. The supply of reserves goes up. To do the opposite, the bank sells bonds it holds and deletes the reserves it receives in payment, draining money back out of the system.
Now connect that to the price. The overnight rate banks charge each other is governed by how plentiful reserves are. Flood the system with reserves and banks, each sitting on more cash than they need, will lower the rate they demand to lend the surplus — just as a glut of any good drives its price down. Drain reserves until they are scarce, and the rate they charge climbs. So buying bonds *adds* reserves and *lowers* the rate; selling bonds *drains* reserves and *raises* the rate. That is the entire mechanical heart of "the central bank cut rates today." No command — just a bond trade large enough to move the supply of reserves to where the target rate clears.
TO LOWER RATES (loosen) TO RAISE RATES (tighten) --------------------------- --------------------------- Central bank BUYS bonds Central bank SELLS bonds -> pays with new reserves -> deletes reserves it is paid -> reserves PLENTIFUL -> reserves SCARCE -> overnight rate FALLS -> overnight rate RISES Reserves are the supply; the policy rate is the price it clears at.
The backstop tool: the discount rate
Open-market operations work on the *whole* pool of reserves at once. The second tool works on a single bank that runs short. The [[discount-rate|discount rate]] is the rate the central bank itself charges when a commercial bank borrows reserves directly from it — the lender-of-last-resort role from the last rung, now used as a routine ceiling. Think of it as the price of the safety valve. If overnight loans between banks ever spiked far above the policy rate, a cash-short bank would simply skip the panic and borrow from the central bank's window instead. That option caps how high the market rate can wander.
Because of that ceiling, the discount rate is usually set a little *above* the policy target, not at it. The idea is to make the window available but slightly costly, so banks treat it as an emergency backstop rather than a cheap everyday source of funds — and so the day-to-day rate-setting still happens through open-market operations. In quiet times almost nobody borrows there; its power is in *existing*. There can even be a stigma to using it, since other banks may read a trip to the window as a sign of trouble — which is exactly why central banks have to design it carefully so a genuinely sound bank is not too shy to use the safety valve when it should.
The blunt old tool: reserve requirements
The third classic tool reaches not into the supply of reserves but into how *much of them banks are forced to hold*. A [[reserve-requirement|reserve requirement]] is a rule that a bank must keep at least, say, 10% of its deposits as reserves rather than lending them out. Recall the money multiplier from the banking rung: a lower required ratio lets each dollar of base money support more lending and so multiplies into more broad money supply, while a higher ratio chokes lending back. Cut the requirement and credit expands; raise it and credit contracts. In principle, this is a direct dial on how far the base gets multiplied.
But here honesty matters more than the textbook diagram. This tool is *blunt*: changing it jolts every bank at once and is too clumsy for the fine, frequent steering that open-market operations allow, so most rich-country central banks barely touch it now. The US Federal Reserve cut its requirement to *zero* in 2020, and several systems have run without binding requirements for years. The reason is a quiet revolution: many central banks now have so many reserves sloshing around (a legacy of quantitative easing, which you will meet later) that the old scarcity story no longer drives the rate at all. Instead they steer the policy rate mainly by setting the interest rate they pay banks on reserves — a newer lever that often does the day-to-day work the textbook still credits to the three classics.
From one tiny rate to the whole economy
Step back and a fair objection appears: who actually cares about an overnight loan between two banks? You will never take one out. The answer is that this tiny, foundational rate is the floor on which every other interest rate is built. A bank deciding what to charge you for a one-year loan starts from what it costs *it* to fund that loan overnight, then adds a margin for time and risk. Nudge the overnight rate and the whole tower of longer rates leans with it. The single price the central bank engineers is the bottom block of a structure that reaches all the way up to your mortgage, the rate on a business expansion, and the yield on a government bond.
This is the bridge to the next guide. Engineering the overnight rate is only step one; the long chain by which it travels out to spending, jobs, and prices is the [[monetary-transmission-mechanism|monetary transmission mechanism]], and it is where monetary policy gets genuinely hard. The links are loose and slow — famously acting with "long and variable lags" of many months — and they can jam. When the policy rate is already near zero it cannot fall much further (the problem you will study as the zero lower bound), which is exactly why central banks reached for newer, more controversial tools like quantitative easing. The toolkit in this guide is how the bank moves its lever; whether the economy actually moves in response is a separate, messier story.