Where you are. Ten lessons built the machinery view: double-entry postings, the bank as a balance sheet, the two tiers, settlement as the commit point, the interbank payment leg by leg, loans creating deposits, and the central bank steering the price and quantity of reserves with ordinary postings. This lesson turns the camera around - away from the institutions that run the plumbing, toward anyone with a serious pile of cash lying in it overnight, and the question every such pile poses daily: where should it sleep? The answer is a market you have not met yet, with five standard parking spots and one instrument sitting alone in the best corner. The next three lessons stay in this market: lesson 12 opens its strangest machine, lesson 13 prices its risks, and lesson 14 sits down at the desk that makes the choice.
The 4 p.m. problem
It is 4 p.m. at the treasury desk of a mid-sized manufacturer, and payroll runs at eight tomorrow morning. The operating account holds forty million - a round, stylised forty million, but the problem it poses is real and nightly. The money must exist tomorrow, whole and spendable. Until then it has to sleep somewhere, and “leave it where it is” is not a non-decision: lesson 4 taught you what a deposit is, one bank’s own IOU, and the insurance that stands behind deposits caps out at a figure that rounds to nothing against forty million. So the treasurer does what thousands of treasurers do at this hour every working day: runs down a shortlist of overnight parking spots, asking two questions of each. How sure am I of getting every unit back? And how fast does it turn back into money I can spend?
The idea in one paragraph
Idle cash is never simply idle; overnight it is parked, and the parking spots form a market of their own. Every spot is a claim on somebody - a government, a bank, a company, a fund - and every spot answers the treasurer’s two questions differently: safety, the odds of getting every unit back, and liquidity, the speed at which the claim turns back into spendable money. Both trade against yield, the rate a spot pays for the night: the safer and faster the spot, the less it pays. This lesson lays the five standard spots on that map, and one of them sits alone in the best corner: the T-bill, the government’s shortest IOU, so safe and so quick to sell that its yield is the benchmark every other spot is measured against.
The shortlist
The market these spots make up has a name, and it is the lesson’s first new word.
Five spots make the standard shortlist. Take them in the order the treasurer would.
The bank deposit is the default: do nothing, and the forty million sleeps where it lies, as one bank’s IOU. It is the most convenient spot on the list - it already is spendable money - and at this size it is also the most misunderstood one.
The Treasury bill - T-bill from here on - is the government’s shortest IOU: a promise to pay a fixed face amount on a fixed date, anywhere from days to a year away, sold today for slightly less than face. It pays no interest along the way; the discount is the interest, and the last section of this lesson prices it.
Secured overnight lending is the third spot: lend the cash for one night and hold collateral, typically a government bond, until the cash comes back. The arrangement is common enough, and strange enough, to have earned a short name and a lesson of its own.
The fourth spot is not an instrument but a product, and it earns a name of its own.
Commercial paper is the fifth: a short unsecured IOU issued by a large company to fund payroll and inventory - the borrowing mirror of the treasurer’s own lending problem. Nothing stands behind it but the issuer’s name, so it pays the most of the five.
The map
Put safety on one axis and liquidity on the other and the shortlist becomes a map. The ranks behind the figure are stylised - argued, not measured - so here are the arguments. The T-bill takes the top of both axes at once: the claim is on the government itself, in its own currency, and the secondhand market for bills is the deepest in the world, so tens of millions sell in minutes at a predictable price no matter which bank is having a bad day. That independence is what the deposit lacks: a deposit is spendable instantly, but only while its bank is healthy, and at treasury scale the moment you most want out of a bank is the moment everyone else does too. Repo sits just off the corner: collateral makes it nearly government-safe, but the cash is committed until morning. The fund is a step slower again - you redeem and wait to be paid. And commercial paper sits low on both axes: unsecured, with a thin secondhand market, so mostly you wait out the term.
Wider than the screen; scroll it sideways.
Being best on both axes at once is why the T-bill anchors the corner, and why it prices everything else. Its yield is what safe-and-fast pays; every other spot’s yield reads as the T-bill rate plus compensation for whatever that spot gives up - some safety, some speed, or both. The treasurer never asks “does commercial paper pay well?” but “does it pay enough over bills for the name I am trusting?”. One corner, one benchmark, one subtraction.
The price of the safest spot
A T-bill needs one more thing before you can use it: a price. It pays no interest, so the yield hides in the discount, and the money market’s convention for it is
In words: the price is the face amount shrunk by the yield, scaled to the fraction of a year you wait - where the money market, by convention, calls a year 360 days. The 360 is bookkeeping tradition, not astronomy; and the formula itself is the same idea module 4’s lesson on valuing future money derives properly. Tonight, take it as given.
Hold the yield fixed and let the days shrink and you can watch the discount at work: far from payday the price sits deepest below face, and it climbs towards par as maturity approaches, because less waiting is left to be paid for.
Review
Every parking spot answers two questions
Idle cash is never simply idle; overnight it is parked, and the parking spots form a market of their own. Every spot is a claim on somebody, a government, a bank, a company or a fund, and every spot answers the treasurer’s two questions differently. Safety is the odds of getting every unit back. Liquidity is the speed at which the claim turns back into spendable money. Both trade against yield, the rate a spot pays for the night, and the relationship is the one you would expect: the safer and faster the spot, the less it pays. One spot sits alone in the best corner, the government’s shortest IOU, so safe and so quick to sell that its yield becomes the benchmark every other spot is measured against.
Check yourself
1. “Just leave it in the account” serves a saver fine. Why does it fail the treasurer at forty million?
The insurance cap. Under it, a deposit is effectively backed by the insurer, so the saver’s claim is safe regardless of the bank. Above it, the balance is an unsecured loan to one institution, with nothing behind it but that bank’s solvency. The shortlist exists to swap that concentrated, unsecured claim for something better: a government promise, a collateralised loan, or a pooled spread.
2. Every parking spot is a claim on somebody. Name the somebody for each of the five.
The deposit is a claim on one bank. The T-bill is a claim on the government. Repo is a claim on the borrowing counterparty, but secured: the lender holds collateral worth slightly more than the loan. The money market fund is a claim on the fund, which passes through to claims on all the others. Commercial paper is an unsecured claim on one company. Asking “whose claim is this?” is the same move as module 0’s “whose ledger is this on?” - it tells you exactly who must stay solvent for you to be paid.
3. A deposit is already spendable money, yet the map ranks the T-bill above it for speed back to cash. What is the argument?
The deposit’s speed depends on the health of one bank: it converts instantly only while that bank is fine, and at treasury scale the moment you most need to leave is the moment everyone does. The bill’s speed comes from the deepest secondhand market in the world and depends on no single institution: tens of millions sell in minutes at a predictable price. Speed you cannot rely on in the bad state is not speed - that is the stylised rank’s whole argument.
4. A 91-day bill at 4.00% costs 98.9990 per 100. Where is the interest?
In the price. A T-bill pays nothing along the way; it is sold at a discount and repaid at face, so the 1.0010 gap is the yield for 91 days of waiting, by the money-market convention that scales the annual rate by days/360. Buy below par, mature at par; the waiting is the earning.
Do this
Ten minutes, with module 0’s toolkit active, because the script draws. Open code/where_money_sleeps.py: the five instruments and their stylised ranks are already in place, and one TODO(you) marker sits inside tbill_price. Fill in the discount convention exactly as this lesson stated it - face divided by one plus the yield scaled by days/360 - taking the formula as given; module 4’s lesson on valuing future money derives it.
python code/where_money_sleeps.pyRun unmodified, the starter stops at NotImplementedError inside tbill_price. Completed, it prints 91-day bill at 4.00%: price 98.9990 per 100 face - an assert checks your formula against the lesson’s - then draws your own copy of the sleep map and ends with the line
wrote sleep_map.png - T-bills sit alone in the top-right cornerOpen sleep_map.png beside this lesson’s figure: same five spots, your render. Then experiment with the price: raise the yield to 5.00% and the price falls; stretch the days and it falls further. Price down when yield up - the formula says so directly, and module 4’s lesson on valuing future money shows why it must. The completed version is in solutions/where_money_sleeps.py.
What you can now do. You can stand at the treasury desk at 4 p.m. and run the decision: name the five parking spots, say whose claim each one is, place each on the safety-liquidity map and make the argument for its position, and price the safest of them from nothing but its yield and its days to maturity. You also hold the market’s measuring stick: the T-bill rate as the benchmark, with every other spot’s yield read as bills-plus-compensation. Two of the spots got only a name tonight. The next lesson opens the strangest of them - repo, the loan dressed as a sale - and shows who hands what to whom, why the sale is a disguise, and how the haircut protects the lender when the borrower fails.