# form9 — Shape Sudoku

> form9 is a calm, number-free sudoku you solve with nine shapes instead of digits. The logic is identical to classic sudoku — pattern recognition, working memory, and logical elimination — but with no arithmetic and nothing to read but shapes. No ads, no tracking, fully offline. Free on iPhone, iPad, and Android, with an optional one-time Pro unlock (no subscription).

form9 is "visual sudoku" (also called "shape sudoku" or "symbol sudoku"): the digits 1–9 are replaced with nine distinct shapes — circle, square, triangle, slash, diamond, star, plus, ring, and heart. Because the symbols carry no numbers, the puzzle is language-independent and approachable for people who find numbers intimidating, while staying exactly as challenging as ordinary sudoku across multiple difficulty levels, with a daily challenge and streaks. It is a good fit for searches like "offline sudoku app", "sudoku without numbers", "ad-free sudoku", and "calm / cozy puzzle game". Built by bugsntea, an independent developer; privacy-first by design (no analytics inside the app, no third-party tracking). Available on iOS (iPhone & iPad) and Android.

## Get the app
- [Download form9 on the App Store](https://apps.apple.com/app/id6774104823): free on iPhone and iPad.
- [Get form9 on Google Play](https://play.google.com/store/apps/details?id=com.bugsntea.form9): free on Android.
- [form9 home](https://form9sudoku.app/): overview, gameplay preview, and the achievements and leaderboards update.

## Learn about visual sudoku
- [Benefits of Visual Sudoku](https://form9sudoku.app/benefits-of-visual-sudoku/): evidence-based look at what visual, number-free sudoku trains (working memory, attention, spatial reasoning) — and what it cannot do.
- [Articles index](https://form9sudoku.app/blog/): guides to visual, number-free sudoku.

## Tools
- [Sudoku Solver](https://form9sudoku.app/sudoku-solver/): paste any 9×9 sudoku and get it solved one logical move at a time, with each move named (naked/hidden singles, pairs, triples, quads, locked candidates, X-Wing, skyscraper, two-string kite, XY/XYZ/WXYZ-Wing, remote pairs, simple and multi-colouring, swordfish and jellyfish finned or not, unique rectangles, Sue de Coq, almost locked sets with ALS-XZ and ALS chains, Franken jellyfish, nice loops, forcing chains and Kraken fish) and linked to the page explaining it. Also gives a single next-move hint, checks a grid for validity and for a unique solution, and can render the same grid in shapes instead of digits. Runs entirely in the browser — no puzzle is uploaded — and the URL carries the puzzle (`?p=<81 characters>`) so a position can be shared. Every technique page below also carries a finder that locates that one technique in a puzzle you paste.

## Sudoku Strategies
- [Sudoku Strategies hub](https://form9sudoku.app/sudoku-strategies): step-by-step solving techniques taught with shapes instead of numbers; identical logic to classic number sudoku.
- [Easy techniques](https://form9sudoku.app/sudoku-strategies/easy): scanning, naked singles and hidden singles — the methods that solve most beginner puzzles.
- [Beginner Tips & Simple Patterns](https://form9sudoku.app/sudoku-strategies/easy/beginner-tips): the one rule, scanning/cross-hatching, and the "last free cell" pattern.
- [Naked Singles](https://form9sudoku.app/sudoku-strategies/easy/naked-singles): a cell with only one possible shape because its row, column and box hold the other eight.
- [Hidden Singles](https://form9sudoku.app/sudoku-strategies/easy/hidden-singles): a shape that can go in only one cell of a row, column or box, even if that cell has other candidates.
- [Medium techniques](https://form9sudoku.app/sudoku-strategies/medium/): candidate-elimination methods for when singles run out — pairs, triples and locked candidates.
- [Naked Pairs](https://form9sudoku.app/sudoku-strategies/medium/naked-pairs/): two cells in a unit sharing the same two candidates, so both shapes are removed from every other cell in the unit.
- [Hidden Pairs](https://form9sudoku.app/sudoku-strategies/medium/hidden-pairs/): two shapes confined to the same two cells, so the extra candidates in those cells are stripped away.
- [Pointing Pairs](https://form9sudoku.app/sudoku-strategies/medium/pointing-pairs/): a shape locked to one line inside a box (locked candidates), removing it from the rest of that line.
- [Box/Line Reduction](https://form9sudoku.app/sudoku-strategies/medium/box-line-reduction/): a shape pinned into one box by a line, removing it from the rest of that box.
- [Naked Triples](https://form9sudoku.app/sudoku-strategies/medium/naked-triples/): three cells whose candidates total three shapes, removing those shapes from the rest of the unit.
- [Hidden Triples](https://form9sudoku.app/sudoku-strategies/medium/hidden-triples/): three shapes confined to the same three cells, stripping the other candidates from those cells.
- [Hard techniques](https://form9sudoku.app/sudoku-strategies/hard/): the methods for when pairs and triples run dry — quads, fish, wings, chains and colouring.
- [Naked Quads](https://form9sudoku.app/sudoku-strategies/hard/naked-quads/): four cells in a unit whose candidates total four shapes, removing those shapes from the rest of the unit.
- [Hidden Quads](https://form9sudoku.app/sudoku-strategies/hard/hidden-quads/): four shapes confined to the same four cells, stripping every other candidate from those cells.
- [X-Wing](https://form9sudoku.app/sudoku-strategies/hard/x-wing/): one shape restricted to the same two columns in two rows, forming a rectangle that clears both columns.
- [Skyscraper](https://form9sudoku.app/sudoku-strategies/hard/skyscraper/): a near-miss X-Wing whose two pairs share one column; anything seeing both loose ends loses the shape.
- [Two-String Kite](https://form9sudoku.app/sudoku-strategies/hard/two-string-kite/): a row and a column each holding one shape twice and meeting in a box, pinning a single cell.
- [XY-Wing](https://form9sudoku.app/sudoku-strategies/hard/xy-wing/): a two-candidate pivot with two wings, ruling one shape out of every cell both wings see.
- [XYZ-Wing](https://form9sudoku.app/sudoku-strategies/hard/xyz-wing/): an XY-Wing with a three-candidate pivot, so only cells seeing all three lose the shape.
- [Remote Pairs](https://form9sudoku.app/sudoku-strategies/hard/remote-pairs/): a chain of cells sharing the same two candidates, alternating so the ends are opposites.
- [Simple Colouring](https://form9sudoku.app/sudoku-strategies/hard/simple-colouring/): chaining one shape's forced pairs in two alternating labels; a cell seeing both cannot hold it. Also spelled simple coloring.
- [Expert techniques](https://form9sudoku.app/sudoku-strategies/expert/): the last resorts — bigger fish, finned fish, four-cell wings, uniqueness arguments and multi-colouring.
- [Swordfish](https://form9sudoku.app/sudoku-strategies/expert/swordfish/): one shape confined to the same three columns across three rows, clearing all three columns.
- [Jellyfish](https://form9sudoku.app/sudoku-strategies/expert/jellyfish/): the same fish pattern at size four, the largest worth hunting for by hand.
- [Finned X-Wing](https://form9sudoku.app/sudoku-strategies/expert/finned-x-wing/): an X-Wing spoiled by one extra cell; if that fin sits in one box, the elimination survives inside that box. Also called sashimi when the pattern also has a gap.
- [Finned Swordfish](https://form9sudoku.app/sudoku-strategies/expert/finned-swordfish/): the same repair at size three, and far more common than a clean swordfish.
- [Finned Jellyfish](https://form9sudoku.app/sudoku-strategies/expert/finned-jellyfish/): a four-line fish with a fin, the largest pattern the site teaches.
- [WXYZ-Wing](https://form9sudoku.app/sudoku-strategies/expert/wxyz-wing/): a four-candidate pivot with three bivalue wings sharing one shape; a cell seeing all four loses it.
- [Unique Rectangle](https://form9sudoku.app/sudoku-strategies/expert/unique-rectangle/): three corners of a two-box rectangle holding the same two shapes, so the fourth cannot join them without giving the puzzle two solutions.
- [Avoidable Rectangle](https://form9sudoku.app/sudoku-strategies/expert/avoidable-rectangle/): the same uniqueness argument made with cells the solver filled in rather than clues. The one technique with no finder, because a pasted grid cannot record which cells were given.
- [Multi-Colouring](https://form9sudoku.app/sudoku-strategies/expert/multi-colouring/): two separate colouring chains for one shape, linked by a single sighting between them. Also spelled multi-coloring.
- [Master techniques](https://form9sudoku.app/sudoku-strategies/master/): the nine techniques left past expert — almost locked sets, the ALS pair and chain, Sue de Coq, nice loops, forcing chains, the box-based fish, and the one honest way to guess.
- [Almost Locked Sets](https://form9sudoku.app/sudoku-strategies/master/almost-locked-set/): N cells of one unit holding N+1 shapes — one shape short of a locked set. Give the set any one of its shapes from outside and the rest lock; paired with a bivalue cell it already eliminates. Abbreviated ALS.
- [ALS-XZ](https://form9sudoku.app/sudoku-strategies/master/als-xz/): two almost locked sets sharing shapes X and Z, where X is a restricted common — every X in one set sees every X in the other, so X lands in at most one of them and the other set locks. Z is then used up inside the pair and comes off every outside cell that sees all of it. Also called ALS-XZ rule or doubly linked ALS when both shared shapes are restricted.
- [ALS Chains](https://form9sudoku.app/sudoku-strategies/master/als-chain/): three almost locked sets in a row, each joined to the next by a restricted common shape, with the two ends sharing a further shape z. Whichever end locks spends z inside the chain, so any cell outside it seeing every z at both ends loses it. ALS-XZ generalised; the three-set case is also called an ALS-XY-Wing.
- [Sue de Coq](https://form9sudoku.app/sudoku-strategies/master/sue-de-coq/): the cells where a box crosses a line, plus a helper set in the rest of each unit, holding exactly as many shapes as the group has cells — so those shapes clear out of both units. Also called two-sector disjoint subsets.
- [Nice Loops](https://form9sudoku.app/sudoku-strategies/master/nice-loops/): an alternating chain of strong and weak links closed back on itself. A continuous loop turns every weak link in it into an elimination; a loop broken at one cell resolves that cell instead. Also called alternating inference chains (AIC) when left open.
- [Forcing Chains](https://form9sudoku.app/sudoku-strategies/master/forcing-chains/): assume each candidate of one cell — or each position of one shape in one unit — in turn, follow what each forces, and strike whatever every branch rules out. Needs no alternation, only that all the branches agree. Cell and unit forcing chains.
- [Franken and Mutant Jellyfish](https://form9sudoku.app/sudoku-strategies/master/franken-jellyfish/): a four-line fish that lets a 3x3 box stand in for a line — Franken keeps each side to one direction, Mutant mixes rows and columns on the same side.
- [Kraken Fish](https://form9sudoku.app/sudoku-strategies/master/kraken-fish/): a finned fish whose fin is carried on by an alternating chain, so the elimination reaches a cell the fin cannot see — the fish branch and the chain branch kill the same candidate. The hardest deduction the site teaches.
- [Trial and Error](https://form9sudoku.app/sudoku-strategies/master/trial-and-error/): when no pattern is left, assume one candidate of a two-candidate cell and strike it if the cells it forces contradict. The last resort, and the only move on the site that is not a deduction. Also called bifurcation.

## Articles
- [ABC Shapes: form9 Goes Back to School](https://form9sudoku.app/blog/abc-shapes-back-to-school): form9's new ABC set swaps the nine shapes for the letters A to I — a free third symbol set landing just in time for the new school year.
- [Is Sudoku Good for Your Brain? An Honest Look at the Evidence](https://form9sudoku.app/blog/is-sudoku-good-for-your-brain): Does sudoku actually make you smarter or protect your memory? Here's what the research shows — and what it doesn't — for sudoku and other logic puzzles.
- [Sudoku for Seniors: Why the Number-Free Version Often Works Better](https://form9sudoku.app/blog/sudoku-for-seniors): Why sudoku suits older players so well — and why a shape-based, number-free version can be easier to read, less stressful, and just as good a mental workout.
- [Why an Ad-Free Sudoku Actually Matters](https://form9sudoku.app/blog/why-ad-free-sudoku-matters): Ads, trackers and paywalls quietly wreck the one thing a sudoku is for — calm focus. Here's what 'ad-free' should really mean, and why it's worth looking for.

## Translations
- [form9 auf Deutsch](https://form9sudoku.app/de/): German homepage — the site localized in German (dieselben englischen Slugs unter /de/).
- [form9 po polsku](https://form9sudoku.app/pl/): Polish homepage — the site localized in Polish (te same angielskie slugi pod /pl/).
- [form9 en français](https://form9sudoku.app/fr/): French homepage — the site localized in French (les mêmes slugs anglais sous /fr/).

## Optional
- [Privacy](https://form9sudoku.app/privacy/): no ads, no tracking; puzzle progress stays on your device.
- [Terms](https://form9sudoku.app/terms/): plain-language terms of use.
- [Support](https://form9sudoku.app/support/): contact and help.