It is a fact insufficiently appreciated by the daylight world that Ankh-Morpork is at least four cities deep.

There is the city you see, and beneath it the city it was built on, and beneath that the city that was built on, all the way down through fire and flood and the sediment of empires to the original riverside village, which archaeologists agree was founded by people who took one look at the Ankh and decided, in defiance of every instinct nature gave them, to stay.1 Cellars open into older cellars. Vaults forget who owns them. And things that wish to be unobserved go downward, on the sound principle that history never audits the basement.

Of-the-Twilight-the-Darkness went downward on a Wednesday night, following a smell.

He had followed it for nine days, in the margins of his bank work, the way a goblin follows anything: patiently, at angles, never along the straight line that the quarry has had time to sweep. The deposits of Amalgamated Civic Holdings (Holdings) Ltd smelled of dust and varnish and uncounted windows, and that smell had a trail, faint as a watermark — through three counting-houses, two lawyers' offices with brass plates so discreet they were practically whispering, and a tobacconist's on Cheap Street that had not, in eleven years of trading, sold a single ounce of tobacco, a fact the goblin established by the simple expedient of smelling that every jar in the window was full of sand.2

The trail went in the tobacconist's front door and did not come out. Which meant it went down.

He returned at midnight with his accountant's satchel, a coil of string, and the small pot that he carried always at his belt — for he was a goblin of the old observance, and the pot was unggue, the pot of that-which-is-owed, and into it, all his life, he had placed a bead for every debt done to his people that the world had declined to write down. The pot was nearly full. He had a feeling, in the manner of goblins, that he was descending toward the place where such accounts were kept.

The cellar beneath the tobacconist's opened, through a door disguised as a wall and a wall disguised as a door, into a stair of old stone that went down past brick, past older brick, past stone the river had licked smooth before the city had a name. And at the bottom, where by rights there should have been darkness, there was light — a flat grey light with no lamp to it, the light of an overcast sky in a place that had never once seen sky — and in the light, a vault.

The real ledgers of the Most Ancient and Honourable Guild of Assessors filled it from flagstone to vault-rib: black-bound, iron-cornered, chained to lecterns like the bibles of a religion whose god was compound interest. Four centuries of the true accounts. Every uncounted window. Every hovel on Scoone Avenue. Every reformer's funeral, entered under Sundries. The goblin's nose read the room the way a scholar reads a library, and what it told him was worth, at a conservative assessment, the entire city, twice.

It also told him — too late, because they had no smell, and that absence was itself the last and worst thing he learned that night — that he was not alone.

The grey gentlemen stood among the lecterns. Six of them. They were reading.

They did not turn pages so much as the pages found themselves turned. They did not move along the rows; they were simply at the next lectern, the way the hour is at the clock. And as each one read, it made, in the margin of the great black ledgers, in ink the colour of fog, a small precise tick.

Of-the-Twilight-the-Darkness, chief accountant of the Royal Bank, holder of the Patrician's commendation, wearer of shoes, stood absolutely still in the manner of his ancestors, who had survived ten thousand years of everyone, and watched the Auditors of Reality approve the books.

One of them spoke. It did not raise its voice and did not need to; the words arrived in the goblin's head with his own thoughts' postmark, which was the worst trick of all.

'You are an unindexed asset.'

'Am goblin,' said Of-the-Twilight-the-Darkness, because among his people, when the darkness asks who you are, you tell it, loudly, so that your gods can find you. 'Am person. Is ruled. Is law.'

'It is ruled,' the grey voice agreed, and a clipboard manifested the ruling, 'and the ruling is an irregularity. Personhood was extended without an adjustment to the underlying inventory. You are carried on one set of books as a person and on another as property. Two entries. One entity. This is double counting.' The heads came up from the lecterns, all six, all at once, with the jump of clock hands. 'We do not correct entries, goblin. Entries are sacred. We correct entities. The reconciliation is scheduled.'

'Which entry,' said the goblin, very quietly, his hand on the pot of that-which-is-owed, 'does you keep?'

'The older one,' said the Auditors of Reality. 'The older entry is always authoritative. That is the first rule of records.'

And Of-the-Twilight-the-Darkness understood, in one cold drop of understanding, what was being done in that vault, and why the ticks were going into the false ledgers, the dirty ledgers, Slope's ledgers — because they were the official ones, the filed ones, the ones the universe had on record — and he turned, and he ran, up through the centuries of brick, with the grey light behind him not pursuing, which was worse, because it meant his departure had already been entered, and somewhere a column was waiting for him to balance.

He took with him no documents. He needed none. He was a goblin: he had smelled every page.

'They're not auditing us against the truth,' said Moist.

It was two in the morning in the attic on Pseudopolis Yard, and the war council of the Office of the Collector-General was convened around Mr Sideways' desk: Moist, Adora Belle, Sideways with his cuff-guards on because standards do not sleep, and Of-the-Twilight-the-Darkness, wrapped in a blanket, his long hands around a mug of banana daiquiri,3 having talked for an hour straight.

'They're auditing us against the files,' Moist went on. He was pacing, which in that attic meant three steps, turn, three steps, like a man rehearsing in a cell. 'Slope's files. Four hundred years of beautiful, official lies. And where reality doesn't match the paperwork, they're not going to fix the paperwork. Gods. It's the perfect bureaucracy. It's what every clerk in history dreams of on his worst night: the file that makes itself true.'

'The golems,' said Adora Belle. Her voice was level in the particular way that meant nothing else about her was. 'On the old books they're property. The old entry is authoritative. Moist, there are two thousand golems in this city, and the grey men are going to rule that the last two decades of them — the freedom, the wages, the Trust, Dorfl's badge — was a clerical error.'

'And error,' said Mr Sideways, who had gone grey himself around the edges, 'gets corrected.' He looked around the attic at his fifty-three years of blue ledgers, the true accounts, the honest city, and for the first time since '73 his voice lost its certainty. 'Mr Lipwig, my books. My books say what's really there. If theirs say different, and theirs make themselves true — sir, what happens to mine?'

Nobody answered. Outside, the rain had stopped, all over the city, all at once, as though it had been told its services were no longer consistent with the precipitation on file.

It was the goblin who broke the silence. He set down his mug, and reached to his belt, and placed on the desk, with the reverence of a priest and the precision of an accountant, the small pot of that-which-is-owed.

'In hole tonight, is learning one thing they is not knowing they teached me,' said Of-the-Twilight-the-Darkness. 'Grey men is saying: older entry is always authoritative. Is their first rule. Is their only rule, maybe.' The vast dark eyes went round the attic, and there was something in them now that had not been there before, something with an edge. 'So. Is simple question for tax man, hey? Guild books is four hundred years old. Very old. Very authoritative.' One long finger tapped the pot, once, and it rang like a small bell in a large temple. 'Goblin pots is ten thousand.'

Moist stopped pacing.

Somewhere below them the city muttered in its sleep, a million people, counted and uncounted, every one of them carried on somebody's books — and Moist von Lipwig stood very still in the lamplight with the expression his wife knew and the Guilds feared and nine dead reformers had never lived long enough to learn, the expression of a man who has just been dealt the bottom card of a very long game, and finds it is the one he palmed there himself, years ago, without knowing why.

'Mr Sideways,' he said softly. 'New ledger. Title page. Write this down: The General Ledger of Everything Actually Owed, Volume One: entries herein supported by instruments of record dating back ten thousand years.' He turned to the window, to the dark city, to the grey corners where the clipboards waited, and he smiled the smile that had sold glass as diamonds and paper as gold and was now, for the first time in its owner's life, entirely sincere. 'If the oldest entry always wins,' said Moist von Lipwig, 'then we are going to out-archive the bastards.'

On Scoone Avenue, at four in the morning, the Vortin residence — one hundred and eighty-two windows, a ballroom, an ornamental lake with its own weather — began, quietly and without fuss, to comply with its assessment.

The night footman heard it first: a soft sound, all along the great façade, like brick remembering. By dawn the east wing had seventeen windows, then eleven, then the smooth blank look of a wall that had never been anything else; the lake's private weather was recalled for review; and the ballroom, in which three centuries of the quality had danced, was found to be a single damp room with a beaten earth floor, much decayed, exactly as described, while the family stood on the lawn in their nightclothes watching their house become its paperwork.

The Times put it on the front page, under the headline that would, in the weeks to come, hang over the whole city like the grey light in the vault:

WHAT ELSE IS IN THE FILE?