What the game is
The Dead Letter Office is battleship sudoku rebuilt as a detective case. Every undeliverable parcel in the city ends its journey at the dead letter office, and its wall of pigeonholes was numbered by the old postmaster to an exacting rule: every row, every column and every bay holds the digits once each — a sudoku, kept as furniture. A smuggling ring has been hiding packets in the wall, in straight bundles of one to four pigeonholes that never touch, and the night inspector has counted the disturbed holes on every shelf and stack.
The case runs in three acts. Restore the smudged board from its surviving digits. Find every packet from the counts, the packet law and the inspector's cross-layer cards. And read the wire: each opened bundle prints its digits onto the tape, and the case's wire key — one of four, from the telegraph card to the spindle rack — turns digit pairs into letters you work out yourself — the ring's final wire, giving the leader's alias, and at higher ranks the drop and the hour as well.
Case procedure
- Start with the digits. Work the sudoku as far as it will go — singles first, then the bay-line and pairs reasoning the higher ranks ask for. Every settled digit is potential packet evidence.
- Read the inspector's cards early. A card like “no packet sits in a pigeonhole showing 2” turns every settled 2 into a cleared hole — and every marked packet into a hole that refuses the digit 2.
- Work the counts. A shelf whose count is met is otherwise packet-free; a shelf with no slack fills itself; and every marked packet clears its diagonal neighbours, because bundles never touch.
- Seal and number the bundles. A bundle only opens when fully marked, sealed off and correctly numbered — the two layers finish together, not in sequence.
- Watch the tape fill. Opened bundles print their digits in wall order. You can start working letters out through the wire key before the tape is complete — pencil each one into its slot yourself.
- Read the wire against the lists. The alias, drop and hour each match exactly one entry in the case file's printed lists — then accuse, with the constables' orders set out in full.
The rulings that matter most
- The postmaster's rule. Every row, column and bay holds each digit exactly once. Digits that clash turn red the moment they appear — the wall teaches its own law.
- The packet law. Bundles are straight runs of one to four pigeonholes and never touch, not even corner to corner. Marks that break it turn red too.
- Counts number pigeonholes, not bundles. A shelf count of 4 might be one long bundle or a run of three and a single, with clear holes between.
- Verified holes are settled. Brass-ringed pigeonholes were checked by the bureau — their packet marks cannot be changed and never lie. Their digits are still yours to find.
- The inspector's cards work both ways. Digits clear pigeonholes of packets; marked packets refuse digits. Each card's fine-print ruling states exactly what it does and does not say.
- Bundles open on merit. Marked, sealed off, and correctly numbered — all three, or the tape stays blank. Clear-marks are your working notes and are never required.
- The wire reads in pairs. How a pair becomes a letter depends on the case's wire key — card rows and columns, the backwards return-post card, hook sums or spindle differences — and the key's own reading line is always printed beside it. The ring's wires always give the alias first.
- No single letter convicts. The printed lists always carry a same-initial rival for the alias and the drop, so the tape must be read in full before the warrant is safe.
Choosing a rank
Sorting Clerk plays on a friendly 6×6 wall — bays of six, digits 1 to 6, four packet bundles and generous surviving digits — a complete three-act case in about ten to fifteen minutes, and the right first night at the office. Night Inspector moves to the full 9×9 wall with a seven-bundle fleet, sparser digits, bay-line reasoning and a wire that names the drop as well as the alias. Postmaster is the long shift: the classic ten-bundle fleet, the sparsest board, naked-pair reasoning, four inspector's cards — and a full wire of alias, drop and hour. Bigger walls add more interlocking deduction, never unfair steps.
Fairness, hints and reveal
Every case is generated and then proved before you see it — on both layers. The number board must be completed by a solver restricted to human techniques at the rank's level, which guarantees a unique solution reachable without guessing; the packet layer must be pinned by the counts, the packet law, the cards and the verified pigeonholes through named deductions alone. When a layout needs more help, the bureau verifies another pigeonhole before the case ships; when it needs too much, the layout is thrown away. Check flags digits and marks that cannot be right without adding any of its own. Hints escalate politely and draw from whichever layer moves next; the entry is always yours to make. Reveal unseals the inspector's ledger and excludes the case from your cases-cracked tally. Wrong accusations — including the right alias with the wrong drop or hour — are stamped on the file; the case stays open.
Printing a case
Any case can be downloaded as a PDF pack built entirely in your browser from the same case number as the on-screen wall: the briefing, the inspector's cards, the ring roster and the candidate lists on the case sheet; one full wall with counts, surviving digits, verified pigeonholes and the wire key sized for pencil work; and an optional answer key showing the completed board, every packet, the tape and the warrant. Sudoku is the most print-native puzzle there is — a pack runs about three A4 sheets and suits classrooms, puzzle clubs and long train journeys, and the case number on the cover reopens the same wall on screen.
Questions from the sorting hall
What is The Dead Letter Office?
The Dead Letter Office is battleship sudoku rebuilt as a detective case, in three acts. A smuggling ring uses the pigeonhole wall of a dead letter office as its post box. First restore the smudged number board — a sudoku, since the old postmaster numbered the wall so every row, column and bay holds each digit once. Then find the ring’s hidden packets battleships-style from the inspector’s shelf and stack counts. Every packet you open prints its digits onto a wire tape, and working the tape out by hand through the case’s wire key decodes the ring’s final wire — the alias of its leader, and at higher ranks the drop and the hour too.
How do the two puzzle layers work together?
They share one wall, and the inspector’s clue cards tie them together: a card like “no packet sits in a pigeonhole showing 9” lets a solved digit clear a pigeonhole of packets — and a marked packet refuse a digit. Bundles also only open when their pigeonholes are both correctly marked and correctly numbered, so the sudoku and the search feed each other all night. The counts, the packet law and the cards are enough to pin every packet by logic alone.
Is there an easier board than the 9×9?
Yes — the Sorting Clerk rank plays on a 6×6 wall with bays of six (two rows by three columns) and digits 1 to 6, with a small fleet of four packet bundles and a generous set of surviving digits. It is a complete case in one sitting: the whole three-act structure — restore, search, decode — in about ten to fifteen minutes. Night Inspector and Postmaster move to the full 9×9 wall.
How does the wire catch the ringleader?
The digits of every packet pigeonhole, read in wall order, form the wire tape. Each case ships one of four wire keys, printed in full in the case file: the telegraph card (first digit picks the row, second the column), the return-post card (read backwards — second digit picks the row), the hook rack (add the pair, take the numbered hook) or the spindle rack (take the smaller digit from the larger). Decoding is your own work — you read the key and type each letter onto the tape yourself. The decoded wire always gives the ring’s alias, then at higher ranks the drop and the hour, each matching exactly one entry in the case file’s printed lists. Accuse the member who answers to that alias, and set out the drop and hour so the constables are waiting.
Is every case guaranteed solvable without guessing?
Yes — on both layers. The number board is proved before you see it: a solver restricted to human techniques (singles, and at higher ranks bay-line interactions and naked pairs) must complete it, which guarantees a unique solution reachable without guessing. The packet layer passes the same standard: the counts, the packet law, the inspector’s cards and a few bureau-verified pigeonholes must pin every bundle through named deductions alone, or the case is rebuilt. An independent brute-force check backs both proofs in testing.
What do the inspector’s cards do?
They are the cross-layer clues — facts the inspector has established that link the digits to the packets, like “no packet sits in a pigeonhole showing 2” or “every two-hole bundle sits on even digits only”. Each card carries a fine-print ruling explaining exactly how it may be used and works in both directions: digits clear pigeonholes, and marked packets restrict digits. They are what makes the wall one investigation rather than two separate puzzles.
What do Check, Hint and Reveal do?
Check flags any digit or mark that cannot be right, without adding anything — it is always safe. Hints come in two strengths and draw from whichever layer moves next: the first press names a deduction and the reasoning behind it; a second press points at the exact pigeonhole, though the entry is still yours to make. Reveal unseals the inspector’s own ledger — board, packets and decoded wire — and a revealed case never joins your cases-cracked tally.
Can I play on a phone?
Yes. The wall sizes itself to the screen, every pigeonhole is a proper button, and the four entry modes — Digits, Notes, Packet, Clear — sit directly beneath the board. Tap a pigeonhole then a number to place a digit, or type it on a keyboard; in Packet and Clear modes a tap marks the hole directly. Your digits, notes and marks save automatically between visits, and each case has a case number so the same wall can be reopened.
Can I print a case?
Yes. Any case can be downloaded as a PDF pack built entirely in your browser from the same case number as the on-screen wall: the briefing, the inspector’s cards, the ring roster and the candidate lists on the case sheet; one full wall with counts, surviving digits, verified pigeonholes and the wire key sized for pencil work; and an optional answer key with the completed board, every packet, the tape and the warrant. Nothing is uploaded anywhere.
Is The Dead Letter Office free?
Yes. All ranks are free to play in your browser with no account or sign-up. A new case is one click away, and every case is generated fresh — the board is built and proved fair, the packets hidden, the cards drawn and the wire enciphered anew, so no two nights at the sorting hall read the same.