What does a central bank actually do?Medium

It sets the price of the shortest money there is, and everything else on this site is priced off that number, at some distance.

4 min read · 706 words

The short answer: it sets the interest rate on the shortest, safest money in its currency, and it stands ready to lend against collateral when nobody else will. Everything else follows from those two, including most of what happens on a screen full of prices.

The rate it actually sets

A central bank does not set mortgage rates, deposit rates or bond yields. It sets the rate at which banks hold reserves with it and borrow from it overnight, and it makes that rate real by being willing to transact at it in unlimited size. From there, everything longer is a market's opinion about where that overnight rate will be in the future, plus compensation for the risks of waiting.

That is why the yield curve is the object to watch rather than the headline decision: the decision moves one point on it, and the rest of the curve moves on what the decision implies about the next two years. A cut that was fully expected can leave long yields exactly where they were, or push them up.

Why it does it

Almost every central bank has a mandate written in law, and price stability is at or near the top of it. The mechanism between the rate and prices is slow and indirect: a higher rate makes borrowing dearer and saving more rewarding, that changes spending and investment over quarters, and that eventually shows up in inflation. Monetary policy is the page on the transmission itself.

Two consequences of that lag are worth holding on to. Policy is set against a forecast rather than against today, so it is always arguable. And communication does part of the work: if a central bank convinces the market that rates will stay somewhere for two years, two-year rates move today without anything being done.

The balance sheet

When the overnight rate is already at or near its floor, there is nothing left to cut, and central banks have used the balance sheet instead — buying bonds to push down longer yields directly, and later selling them or letting them mature. The instrument being bought is usually a government bond, and the money used to buy it is reserves the central bank creates.

The point that gets lost: this changes who holds which asset, not how much anybody owes. A pension scheme that sold a bond now holds cash, and something has to be done with it.

Lending when nobody else will

A solvent bank can still run out of cash, because its lending is long and its funding is short — that mismatch is what a bank is. Against that specific failure, the central bank lends: against collateral, at a rate, to an institution it judges solvent.

The whole design of the plumbing rests on this. Repo is the market where cash and collateral trade against each other every day, and the central bank is the backstop underneath it. Central banking is the seat, and reading a statement is the playbook for what one actually says.

Supervision, which is a different job in the same building

In many jurisdictions the central bank also supervises banks: capital, liquidity, governance, stress tests. It is a separate mandate with separate people and often separate legal powers, and confusing the two leads to bad readings of the news — a supervisory action against one bank is not monetary policy, and a rate decision says nothing about any individual firm. Supervision is the seat.

What it cannot do

It cannot fix a price it has not committed unlimited resources to defending, and defending one has a limit that is visible only afterwards — the Swiss franc floor in 2015 is the case study. It cannot make a bank solvent by lending to it, only liquid. And it cannot decide who gets credit; it sets the price of the shortest money and the rest of the system decides where that money goes.

The one sentence to take away

A central bank controls one very short rate and the willingness to lend against collateral; everything longer than overnight is the market's forecast of that rate plus a payment for waiting, which is why the curve tells you more than the announcement does.