A yin yang book punishes a bad file in a way a sample puzzle never shows. Your buyer judges puzzle forty-one, when a corner they filled white turns out to cut the black cells into two separate groups, or a neat 2x2 square of black sits in the middle of their finished board and they cannot tell whether they made it or the puzzle forced it, and the solver who trusted the two simple rules concludes the fault is theirs. Yin yang looks like the gentlest puzzle in the world: a rectangular grid, some cells already carrying a black or a white circle, and nothing else β no numbers, no lines, no regions. The solver fills every empty cell with a black or a white circle. That is the whole surface. Underneath it is a contract in five parts, each part exact. First, the fill rule: every cell gets exactly one colour, black or white, and a circle printed as a given never changes colour. Second, the black rule: all black cells form one single connected group, joined edge to edge β up, down, left, right β never only diagonally. Third, the white rule: all white cells do exactly the same, one single connected group, joined edge to edge. Fourth, the window rule: no 2x2 block of cells, anywhere in the grid, is all black or all white. Fifth, tying it shut: exactly one filling, reachable by deduction alone, with no guessing. A puzzle that breaks any part is not slightly harder. It is false as stated. A good yin yang book is a verified board: every puzzle re-checked colour by colour and window by window from its own key, puzzle by puzzle.
Two warnings belong at the front, because they are the mistakes this genre makes most often, and both start with puzzles yin yang is not. Yin yang is not nurikabe and it is not kuromasu. Those puzzles also trade in black and white cells, but they carry numbers, islands, and counts, and they usually ask for only one colour to stay connected while the other is deliberately split apart. Yin yang has no numbers at all, and it is perfectly symmetric: black must stay in one piece and white must stay in one piece, at the same time, on the same board. A rules page borrowed from a shading puzzle β talk of islands, walls, or numbered cells β teaches the buyer a puzzle the pages do not contain. The second warning is about the 2x2 window, in both directions. The stated rule bans a 2x2 block that is all one colour. It does not ban a 2x2 block that alternates, black-white-black-white like a tiny checkerboard β but a checkerboard window can never appear in a finished yin yang board anyway, because its black cells touch only diagonally, and so do its white cells, and diagonal contact does not connect anything. An alternating window is therefore impossible for a deeper reason, connectivity, and a setter who treats it as allowed in a key has shipped an unsolvable board, while a setter who lists it as a separate stated rule confuses buyers about which rule did the work. Say both things cleanly on the rules page: no solid 2x2, by rule; no alternating 2x2, by consequence. And say what connected means, in words a first-time buyer cannot misread: sharing an edge counts, touching at a corner does not.
The book is the whole board, not the sample puzzle
Buyers do not buy a circle style. They buy the promise that every puzzle in the file obeys the rules printed at the front, at the difficulty the cover claims, in the order the book presents them. A free sample almost never breaks that promise, because a sample is the one puzzle the setter checked three times. The breakage lives at puzzle forty-one, in the middle of the book, where a generator exported the givens from one board and the key from the next, or a hand-setter flipped one given from white to black to fix a corner, quietly disconnected a whole region of the other colour, and never re-ran a single check. That is why the unit of quality in a puzzle book is the whole set, verified as a set: the same checks, run the same mechanical way, on every single puzzle, with the results kept. If you generated the puzzles, keep the generator's settings per puzzle, including its grid size and given count. If you set them by hand, keep your solving notes. When a buyer writes in about one puzzle β and in this genre they write in with the page number and the corner they cannot make connect β you want to answer about that puzzle, not go hunting through a file you can no longer reconstruct.
The rules page is part of the product, not decoration. State all five parts of the contract in plain language, with one small worked example showing a short board being filled cell by cell: one 2x2 window where three cells of one colour force the fourth to be the other colour, one passage where a colour is about to be cut in two and the connecting cell is forced, and the board finishing with both colours traced, edge by edge, as single groups. That worked example is the one idea a new buyer must carry away, because yin yang is solved against connectivity, not just against the windows, and it is the idea a short rules page most often drops in favour of a pretty picture of a finished board with no group ever traced. Most one-star reviews in this genre are not "too hard" or "too easy". They are "the finished board breaks the rules" β and that mismatch is almost always a setter who held the real board in their head, or in the generator, and printed givens that were never re-checked against their own key.
The contract, part by part
The fill rule is the frame everything else hangs on, so check it like one. Every cell in the finished board carries exactly one colour, every printed given keeps its printed colour in the key, and no cell is left blank, doubled, or changed. Audit it mechanically: extract every given from the puzzle page, look up that cell in the key, and record that the colours match, one line per given, no sampling. A single flipped given does not make a slightly different puzzle β it usually makes a puzzle with no solution, or with a different one, while the printed page still promises the original.
The two connectivity rules are the heart of the puzzle, and they admit no exceptions and no diagonal shortcuts. Trace the black cells of the key as a graph, moving only up, down, left, and right: starting from any black cell, you must be able to reach every other black cell without leaving black. Then do exactly the same for white. A board where black is one group but white falls into two groups has failed, however elegant the black shape looks, and a board where two same-colour regions touch only at a corner has failed too β corner contact is precisely the trap this puzzle sets, because the eye reads a diagonal cluster as connected and the rules do not. These two checks are purely structural and catch a whole class of generator and transcription errors before any solving is attempted.
The window rule is the local rule, and it is checked locally. Slide a 2x2 window across every possible position in the finished board β every row pair, every column pair, no gaps at the edges β and record the four colours in each window. None may be all black, and none may be all white. That scan sounds trivial, and it is, which is why there is no excuse for skipping it: it is a finite, countable list of windows, and a book can state exactly how many windows each puzzle contains and that every one was checked. Remember the consequence alongside the rule while you scan: a window that alternates is not banned by this rule, but it cannot survive the connectivity check, so its presence in a key is still a failed key, caught by the trace above.
Uniqueness is the part of the contract a plausible board cannot promise. A set of givens can usually admit more than one filling that respects connectivity and the windows if the setter only checks that a filling exists β sparse givens in open corners are where second fillings hide β and a puzzle that fills two ways tells one of its two correct solvers they are wrong. The fifth part therefore has to be proven, not assumed: an independent solve, or a solver run, that counts fillings and returns exactly one, with that one reachable by deduction rather than by trying a colour both ways until one branch survives.
Verification: flood-fill both colours, then scan every window
Verification is a separate pass from solving, done mechanically, ideally by a different method than the one that made the puzzle. Solve each puzzle once as a buyer would, by deduction only, and keep the solve honest: if you guessed a cell, and the guess worked, that puzzle has not been shown to have one solution, and it goes back. Then run the structural audit on the key, in this order. One: inventory audit β every given on the puzzle page appears on the key page, in the same cell, with the same colour. Two: completeness audit β every cell of the key carries exactly one colour, with no blanks and no cell carrying both. Three: black trace β flood-fill the black cells moving only edge to edge, and confirm the fill reaches every black cell and stops, one group exactly. Four: white trace β the same flood-fill for white, one group exactly. Five: window audit β every 2x2 window in the board is listed and checked, none all black, none all white. Six: alternating-window audit β no 2x2 window alternates like a checkerboard, because such a window would already break both traces. Seven: uniqueness audit β a second, independent solve, or a solver run, that counts fillings and returns exactly one. Eight: consistency β puzzle page, key page and any thumbnail or marketing sample of that puzzle are the same puzzle, with the same givens in the same cells and the same colours in those cells.
Keep the audit results with the book's files, puzzle by puzzle. When a corrected edition ships, that record is how you know which puzzles actually changed and which were only suspected. It is also how you answer the buyer who found a real error: not with an apology and a shrug, but with the corrected puzzle, the audit lines that now pass, and the corrected file sent to everyone who bought the book β which is where the delivery side of this business, covered below, stops being administration and becomes the difference between a one-star review and a loyal repeat buyer.
Difficulty is connectivity pressure and window pressure, not just grid size
Grid size is the weakest difficulty lever in yin yang, and the one covers lean on hardest. A large grid whose givens are dense, with windows that force cell after cell in chains, falls colour by colour in a satisfying sweep, while a small grid whose givens are sparse and badly placed, leaving both colours free to split in several places at once, can stop an experienced solver cold. The real levers are these. Window pressure: how many 2x2 windows start with three givens, or reach three filled cells early β three of a colour in a window force the fourth, so a book can be graded by how often that forcing move is available and how far each forced cell cascades into neighbouring windows. Connectivity pressure: how often a colour is threatened with being cut in two, so the solver must find the one connecting cell before anything else can proceed β puzzles where both colours are threatened at once, in different corners, are genuinely hard at any size. Given placement: givens clustered in one area leave the rest of the board under-determined and guessy, while givens spread so that each region constrains its neighbours produce deduction from start to finish. Edge and corner behaviour: colours running along a border have fewer routes to stay connected, so border givens do more work than central ones, and a setter who grades by given count alone will misjudge every border-heavy puzzle in the set.
Grade a book by testing solves, not by measuring grids. Time honest solves across the set, note where each puzzle's deductions stall and restart, and order the book so difficulty genuinely climbs β early puzzles with dense givens, frequent three-in-a-window forcing, and one threatened colour at a time, later puzzles with sparse givens, both colours threatened together, and connections that only resolve near the end. Print the grading honestly: "easy" should mean the forcing moves mostly present themselves, not merely that the grid is small. A buyer who trusts your grading buys the next volume; a buyer who was ambushed by a connectivity knot on page three of the easy book leaves a review about that instead.
Print honesty: the page is the interface
A yin yang page is read in pencil first and pen last, and the print has to survive both β starting with its circles, because black, white, and the grid between them are this genre's whole interface. A black circle must read as unmistakably black and a white circle as unmistakably white in greyscale, after a photocopy, at arm's length: a white circle is an outline, not an absence, so its ring must print strongly enough that a buyer can tell a printed white given from an empty cell they have not filled yet. That distinction β given white, given black, still empty β is the single most common print failure in this genre, and it makes a puzzle unsolvable as printed because the buyer is solving a different set of givens from the one the setter checked. Givens and buyer-filled cells must also be distinguishable in your design, by weight, ring, or shading, so a solver can always tell what they may not change. Your key must print the finished board in the same two unmistakable colours, at the same size relation to the puzzle page, so a buyer can trace either colour across the key cell by cell instead of taking the picture on faith.
Proof the final PDF, not the working file. Export at the exact trim size, print one puzzle and its key on a home printer at 100 percent, and solve the printed puzzle with the printed rules page as your only reference. Check the three things screens hide: whether given white circles still read as givens, not blanks, after the page has been photocopied once β classrooms and care settings will photocopy it β whether given black circles and buyer shading remain tellable apart on that same copy, and whether there is room in each cell for the small trial marks solvers actually make before committing to a colour. If a puzzle needs a screen's zoom to tell a white given from an empty cell, it is not ready to be a download.
Licensing, provenance, and the download itself
State the licence where the buyer decides, not buried after purchase: who may solve the puzzles (one person, a household, a classroom, a club, a care setting), how many copies they may print, whether a teacher may hand a page to thirty students, whether a club may run one puzzle as a group activity, and whether the file itself may be shared, resold or republished β and say plainly that sharing the file is not the same as sharing a printed page. Puzzle books are bought heavily by teachers, club organisers and carers, and those buyers are trying to comply; a licence they cannot find reads to them as a licence that says no. Say how the book was made, too: hand-set, software-generated and hand-checked, or software-generated and mechanically audited β and state your connectivity convention in the book and in the listing, that groups join edge to edge and never only diagonally, because a buyer raised on a puzzle that counts diagonals will otherwise judge your correct boards as wrong ones. Provenance is not an apology β in this genre it is a quality claim, as long as it is specific. "Generated, then both colours flood-filled from their own keys, and every 2x2 window in every puzzle scanned" is a claim a buyer can weigh; "high quality puzzles" is not.
Delivery is the last part of the contract. The buyer should get the file immediately, from a link that works the first time, on the device they bought it on, with re-downloads available when β not if β they lose it. Send a corrected edition to every past buyer when one ships; do not wait for them to discover the errata the hard way. I run my own download delivery through FileFreight β flat monthly pricing instead of a cut of every sale, expiring links, and updated files reaching past buyers without me re-emailing attachments by hand β but whatever you use, test it as a buyer: pay, receive, download, lose the file, recover it, and receive an update, before you ask strangers to trust it with money.
The launch checklist
- Every puzzle solved by deduction alone β no guessed cell in any solve, start to finish.
- Trace audit passed: black flood-fills as one edge-connected group and white flood-fills as one edge-connected group, on puzzle keys alike.
- Window audit passed: every 2x2 window in every puzzle checked, none all one colour, none alternating.
- Given audit passed: every printed given appears in the key in the same cell with the same colour, and no given was flipped or moved.
- Uniqueness audit passed: exactly one filling satisfies the fill, connectivity, and window rules, and it is the filling printed in the key.
- Print proof passed: white givens unmistakable from empty cells after one photocopy, black givens tellable from buyer shading, on a home-printed page at final size.
- The errata rule honoured in advance: any disconnected colour, any solid 2x2 window, any flipped given or any forced guess means a corrected file goes to every buyer, not just the one who noticed.
