A Lockout book punishes a bad file in a way no sample puzzle ever shows. Your buyer judges puzzle forty-one, when a line your key swears is sound runs a 2 in one diamond and a 7 in the other, and a quiet 5 sits in the middle of the line — not touching either diamond, breaking no row, column or box, and forbidden absolutely, because on a lockout line no digit may be equal to or between the digits in the diamond ends — or when a pair of diamonds only three apart, a 4 and a 7, anchors a line whose law demands at least four, so the book is false before the buyer places a single digit — or when diamonds of 1 and 9 cap a line with cells between them, a line no digit in the language can legally occupy. Lockout Sudoku, in the form this book sells, is a classic frame with a law of exclusion: a 9 by 9 grid, digits 1 to 9 once in every row, column and outlined 3 by 3 box, crossed by dark lines that end in diamonds, and on every line the two diamond digits differ by at least four while every other digit on the line lives strictly outside the interval they span. Underneath it is a contract in five parts, each part exact. First, the row rule. Second, the column rule. Third, the box rule. Fourth, the lockout rule: read the two diamonds first, because they ban a whole band of values — the smaller diamond, the larger diamond, and everything between them — from every remaining cell of the line. Fifth, closing the contract, given integrity with uniqueness: the givens are fixed exactly as published, and exactly one completed grid satisfies every part at once, reachable by deduction alone, with no guessing. A puzzle that breaks any part is not slightly harder. It is false as stated. A good Lockout book is a verified stack of intervals: every line's band re-computed from its diamonds, on the final file, puzzle by puzzle.
Four warnings belong at the front, because they are the mistakes this genre makes most often, and all four start with puzzles Lockout is not. Lockout is not a Between Line — it is a between line turned inside out, and the geometry is the trap: a between line, capped by circles, demands that every digit on the line lie strictly between its two end digits, and a lockout line, capped by diamonds, demands that no digit on the line lie between its two end digits, or equal them; the two laws share a picture and forbid each other's solutions, and a checker, a generator or a rules page that confuses them produces a book that is wrong on nearly every line while passing its own audit. Lockout is not a German Whisper, though both laws push digits apart: a whisper constrains neighbours, demanding adjacent digits differ by at least 5, and has no endpoints and no banned band — a whispered line can wander through the middle values a lockout line forbids, provided each step is wide, and a lockout line can sit perfectly still in value, repeating one small digit, provided it never enters the band. And lockout, in its standard form, bans the diamond digits themselves from the line's other cells: some summaries of the dark line say only that digits may not fall between the diamonds, and setters will find that shorter sentence if they go looking; the standard rule as the puzzle portals state it — digits on a lockout line must not be between or equal to the digits in the diamond ends — bans the endpoints too, and a book generated under the shorter reading lets a diamond's digit reappear mid-line in another house and must not be sold under the standard name. Finally, lockout does not ban repeats: neither portal wording forbids a digit from appearing twice along a line where the classic rules allow it, and a generator that bans repeats on top of the band is solving a stricter, different variant whose uniqueness proofs will not transfer. A buyer who has been taught any of these four impostors by a careless rules page will solve your book under the wrong law and write the review your book did not earn.
The book is every banned band, not the sample puzzle
A sample puzzle proves almost nothing about a Lockout book, because the law lives in the interval, and an interval is invisible until somebody computes it. Two diamonds, a 2 and a 7, silently ban five values — 2, 3, 4, 5 and 7, with 6 banned as well for sitting between them — from every other cell of their line: six of the nine digits in the language, gone, on a line whose remaining cells may hold only 1, 8 or 9. No house in the grid announces that ban. It does not appear in any row, column or box, and a digit placed inside the band breaks nothing a classic checker can see. That is the shape of the buyer's experience: they do not solve your sample, they solve your fortieth line at midnight, pencil in hand, checking a band your file may never have computed. When the band fails, the buyer does not think the puzzle is hard. They think the book is careless, and they are right, because the band is not an edge case of the product. The band is the product, repeated hundreds of times per book.
So the unit of quality in this genre is not the puzzle and it is certainly not the listing. It is the line read as two diamonds and a banned interval, computed and verified: every line, every diamond pair, every interior cell, against the solved key and against the printed page. Everything below — the contract, the audit, the difficulty scale, the print rules — is machinery for keeping that one promise at book scale, because a book is where exclusion stops being a clever constraint and starts being a manufacturing problem: hundreds of lines, thousands of banned placements, one standard.
The contract, part by part
Part one: the row rule. Every row of the 9 by 9 grid holds the digits 1 to 9 exactly once. No lockout line, however wide its diamonds stand apart, excuses a doubled digit in a row, and no buyer forgives it: the classic frame is the floor the whole book stands on.
Part two: the column rule. Every column holds the digits 1 to 9 exactly once. A line that runs down a column shares cells with that column's own law, and the two laws agree on repeats inside the column and part company everywhere else — the column cannot see the band a distant diamond pair imposes on a cell, and the checker that leans on the column to police the line will miss exactly the placements the line exists to forbid.
Part three: the box rule. Every outlined 3 by 3 box holds the digits 1 to 9 exactly once. Lines wander through several boxes, diamonds sit in one box while their band governs cells in three others, and most of the placements a lockout line forbids are placements no single box ever judges; the box rule is necessary, it is nowhere near sufficient, and a book audited only house by house has not begun to audit its lines.
Part four: the lockout rule. Take one dark line and read its two diamond cells first, because everything else on the line is judged against them. Two demands fall on the line, and both are total. The diamond digits must differ by at least four: a pair three or fewer apart anchors nothing, and a puzzle containing one is broken before solving begins. And no other digit on the line may be equal to or between the two diamond digits: if the smaller diamond is the low end and the larger is the high end, every remaining cell of the line must hold a digit below the low end or above the high end — the whole closed interval between them, endpoints included, is locked out. Four consequences follow, and a setter who cannot recite them is not ready to generate a book. First, the count of surviving values is fixed by the gap alone: the line's interior may use only the digits below the low diamond plus the digits above the high diamond, and that total is eight minus the gap — a gap of four leaves at most four legal values, a gap of five leaves three, a gap of six leaves two, and a gap of seven leaves exactly one. Second, the extreme pair is a trap with teeth: diamonds of 1 and 9, a gap of eight, leave no legal digit at all for any other cell, so a lockout line capped by 1 and 9 can consist of its two diamonds and nothing else — a third cell on such a line is not a hard puzzle, it is a printing error with a cover. Third, a gap of seven forces the interior outright: diamonds of 1 and 8 leave only 9, and diamonds of 2 and 9 leave only 1, so every interior cell of such a line holds that single digit wherever the classic rules permit it to stand. Fourth, there are only fifteen legal diamond pairs in the language — five pairs four apart, four pairs five apart, three pairs six apart, two pairs seven apart, and the single pair eight apart — and a setter can hold the whole table in view while generating, which means a sixteenth pair in a finished book is never a subtle error; it is an unaudited one.
Part five: given integrity and uniqueness. The givens printed in the book are the givens the solver proved unique — no digit added for looks, none removed for elegance after the proof. Exactly one completed grid satisfies all five parts at once, with the lockout law switched on, and that grid is reachable by deduction alone. The control matters as much as the proof: run the same givens with the dark lines ignored and the classic frame will often admit the intended grid among several, which is fine and expected — the lines are load-bearing — but a grid that stays unique only while a checker forgets the equal-to clause, or forgets that the diamonds must stand at least four apart, is a grid whose proof was run under a weaker law than the rules page promises, and its second solution is waiting for a buyer with a pencil.
Verify like a buyer: the eight-step audit
Run this audit on the final exported file — the file the buyer downloads, not the generator's working grid — and keep the record per puzzle. A buyer checks bands. So does this audit, in an order that catches the cheap failures before the expensive ones.
Step one: line inventory. List every dark line in the puzzle: how many lines, how many cells each, which cells in reading order, and where the two diamonds sit. Count the cells twice, once from the puzzle page and once from the key. Any line whose diamonds are missing, doubled at one end, or drawn on cells the key does not list fails here, and any line whose cells on the page differ from the cells in your working file fails with it. Most book-scale errors are born at this step and discovered by buyers because nobody ran it.
Step two: diamond extraction. For each line, read its two diamond digits off the solved key and write the pair down, smaller first. This pair — not the line's path, not its interior — is the object the law judges first. An audit that works along the line cell by cell before recording the diamonds is auditing in the order the eye travels, not the order the law applies, and will check placements against a band it never actually computed.
Step three: gap check. Subtract the smaller diamond from the larger. Any gap below four fails the line on the spot, whatever its interior holds, and the failure is the book's, not the puzzle's: fix the generator's pair table — the fifteen legal pairs, no others — and regenerate, rather than patching a single line by hand.
Step four: band computation. Write out the banned interval explicitly: the low diamond, the high diamond, and every value between them. Then write the surviving values beside it — everything below the low end, everything above the high end — and check the count against the gap: survivors must number eight minus the gap. A line whose computed survivors disagree with that arithmetic is a line whose diamonds you misread; go back to step two, because one of the two readings is wrong, and the buyer's pencil will find which.
Step five: interior scan. Walk every non-diamond cell of the line on the key and test its digit against the banned interval. One digit inside the band — including a digit equal to either diamond, standing in a different house where the classic rules raise no objection — fails the line. This is the step the genre lives on: it is the only step that looks at exactly what the buyer will look at.
Step six: reverse recount. Close the key, open the printed puzzle page, and recount every house and every line from the page alone, diamond pairs re-read from scratch. Then compare against step two's pairs, digit for digit. The reverse pass exists to catch the export step — the shifted line, the stale key, the diamond redrawn onto a neighbouring cell — and it only works if it is genuinely run from the page, not from memory of the file.
Step seven: uniqueness, law on, plus the impostor controls. Prove the puzzle solves uniquely with the lockout law enforced in full — gap at least four, band closed at both ends. Then run two controls that protect you from your own toolchain: once with the between reading, the exact opposite law, and once with the shortened reading that bans only what lies strictly between the diamonds and lets the diamond digits reappear inside. If the solution changes under either control, your main proof was only as strong as the law your solver actually implemented, and the settings — not the puzzle — are what need fixing before the book ships.
Step eight: the print proof. Print one full page at final size, in greyscale, on the cheapest paper your buyers plausibly own. Dark blue is a treacherous colour: it prints near-black, it can swallow a thin line into the grid ruling, and a diamond a buyer cannot find is an endpoint they cannot read. Every line must stay traceable from end to end, both diamonds must read as diamonds — not as circles, which would promise the opposite law, and not as squares, which promise parity — and the whole figure must be distinguishable from any neighbouring line by geometry and weight, not by colour alone. If the diamonds vanish, the book fails step eight no matter how clean its bands are.
Difficulty is interval work, not given counts
Buyers forgive a hard Lockout book. They do not forgive a dishonest one — a book labelled gentle whose third puzzle walls them for an hour, or a book labelled fierce that folds in ten minutes. In this genre the given count is the weakest lever on the shelf, because the law's power concentrates in the gaps, not in the clues. Grade difficulty by the interval work the solve actually demands, and name the levers in the listing.
The first lever is gap width: how much of the number line each diamond pair removes. A gap of four bans five values and leaves four standing, a firm nudge; a gap of seven bans eight values and leaves a single survivor, which turns every interior cell of that line into the same forced digit and can detonate a whole region of the grid the moment the diamonds are read. Books built on wide gaps solve as cascades; books built on fours solve as sieges. The second lever is interior length against survivor count: a long line with few survivors concentrates the law, because many cells must share very few values across several houses at once, while a short line over a wide band barely speaks. The third lever is band position: a pair like 4 and 8 locks out the middle and top-middle of the range and leaves only the extremes, 1, 2, 3 and 9, so the line's cells argue with every mid-range given in their houses — count how often the band, not the classic frame, delivers the decisive elimination in your own test solves, because that count, honestly taken, is the difference between a Lockout book and a classic book wearing diamonds. The weakest signal, as always, is the raw number of givens, and a listing that grades by it alone is grading by the one number the buyer cannot feel.
So test-solve the whole book, in order, timed, from the printed pages — not from the generator's own estimates, which know the solution too well to judge the search. Grade in bands a buyer can plan an evening around, state the levers you used on the listing page, and let the hardest puzzle in the book set the label: one wall puzzle in a gentle book earns the review, and it will be quoted back at you by name and number.
Print honesty: lines a buyer can actually follow
A Lockout book sells an interval law printed as a line with two diamonds, and the buyer's whole experience of the law runs through the drawing of that figure. Print is not decoration here; it is the rule made visible, and it has failure modes a screen proof will never show you.
The figure must read as one object with two unmistakable ends. A lockout line is a band wearing a path: the buyer must be able to see, at a glance and in bad light, exactly which cells belong to it, and which two cells carry the diamonds — because a cell wrongly included invents a ban the law never made, a diamond wrongly read moves the band, and a diamond missed entirely deletes the law from that line. Draw diamonds large enough to survive reduction, with a weight and outline that survive greyscale printing; dark blue on white, printed cheaply, can collapse into the same grey as the grid, so separate lines by geometry and pattern, not by shades of a colour that prints as one grey. Keep line colour out of the rules text as the only identifier — say dark line with diamond ends and describe the drawing, so a buyer printing in black and white loses nothing but the colour. Never let a diamond be confusable with a circle or a square at final size: those shapes carry other laws, one of them the exact opposite of yours, and a buyer who reads your endpoint as a between circle will solve against you all evening. Puzzle page and answer key must draw the identical lines and identical diamonds over the identical grids; a key that redraws its figures from a second export is a second book, unaudited, hiding at the back of the first. And give every line's cells room: cramped grids shrink digits, shrink diamonds into blobs, and turn the band-checking the genre lives on into eyesight work the buyer never agreed to do.
Licensing, provenance, and whose puzzles these are
A Lockout book is easy to assemble dishonestly and hard to assemble well, and the difference is records. Variant sudoku has a living community of setters whose puzzles circulate in packs, portals and streams, with the dark line's standard wording attached. A puzzle lifted from a portal, re-drawn in your template and sold under your cover is not made yours by the re-drawing, and this community is small enough — and archives its work carefully enough — that provenance is not a courtesy. It is a survival trait.
Keep three records per puzzle and the book can defend itself. The generation record: how this grid and these lines were produced, by what tool, under what constraint settings — including, in writing, which lockout reading the generator enforced, the standard closed-band law with the four-apart minimum or one of the shorter summaries — because that one setting is where this genre's books quietly diverge. The rights record: written licence or your own authorship, in a form you could show a marketplace without embarrassment. The audit record: the eight steps, dated, against the final file, with the diamond pairs and banned bands kept so a disputed line can be re-walked digit by digit. When a marketplace asks where a book came from — and marketplaces increasingly ask — the seller with records answers in minutes. The seller without them discovers that "my generator made it" is a claim, not a record.
Corrected files, errata, and the delivery rule. Every puzzle book ships with a mistake eventually; the genre only decides whether the mistake becomes a one-star review or a five-star story about a seller who fixed it. The rule is simple and it is not negotiable: when any band in any puzzle is wrong — a diamond pair three apart in a key, an interior digit sitting inside its line's interval, a third cell on a line capped by 1 and 9 — every buyer gets the corrected file, not just the buyer who complained, and the correction goes out through the same delivery that sold the book, so it lands in the same library, the same downloads page, the same inbox thread the original came from. This is where delivery stops being plumbing and becomes part of the product: a download tool that versions your file and re-delivers the corrected edition to past buyers turns an errata notice from a hope into a fact, and it is why I deliver my own puzzle books through FileFreight — one flat monthly price for download delivery and license keys, the corrected file replaces the old one for every buyer at once, and nobody has to write in and ask. Pair the corrected file with a plain errata note inside it: puzzle number, what was wrong, what changed, in words a buyer can check against their printout. Never silently swap a file a buyer may already have printed and half-solved; a silent fix gaslights the exact buyer who found your error, and they are the buyer your next review is made of. Keep the old version archived, keep the note dated, and let the record show the book getting truer over time.
The launch checklist
Before the listing goes live, walk this list in order and do not skip backward. One: the rules page declares the law — 9 by 9 grid, digits 1 to 9, dark lines ending in diamonds that differ by at least four, no other digit on a line equal to or between its diamond digits — and names the neighbouring variants, between lines above all, only to exclude them. Two: every puzzle has passed the eight-step audit on the final exported file, with a written per-puzzle record and its diamond pairs and bands kept. Three: no line in any key holds a diamond pair under four apart or an interior digit inside its band, verified line by line, pair by pair. Four: every gap-seven line's interior is the single surviving digit, no line capped by 1 and 9 carries a third cell, and forward and reverse recounts agree on every house and every pair. Five: difficulty is graded from real solves, and the listing names the levers used. Six: puzzle pages and keys present the identical lines and diamonds, and every diamond survives printing at final size in greyscale. Seven: all puzzles are your own generation or licensed, records kept, and the generator's lockout setting is written down. A book that can say all seven, truthfully, has earned its listing copy — and the copy should then say the plainest true thing about it: these bands were recomputed, one by one, twice.
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