Can Every Sudoku Be Solved Without Guessing?
Learn the difference between blind guessing, Trial & Error, and a temporary assumption used to produce a reproducible contradiction proof.
Read the guideLogic First · No Guessing
Sudoku Solving Guides
Sudoku Paradise explains not only what a technique eliminates, but why the elimination must be true. This growing library follows the same human-solvable doctrine used to evaluate and certify our puzzles.
Hardest Sudoku Compared
AI Escargot, Everest, and the Golden Nugget face the same 54-solver logic deck. See why three Monsters produce three kinds of difficulty.
Read the benchmarkHow Sudoku Difficulty Is Measured
Why the techniques required to solve a grid matter more than its givens count.
How Difficulty Is MeasuredLocked Candidates
Pointing and claiming interactions between boxes, rows, and columns.
Locked Candidates LessonPairs, Triples, Quads, and intro to Uniqueness
Learn how Naked and Hidden Pairs, Triples, and Quads restrict candidates, reveal one another, and lead toward more advanced Sudoku strategies.
Pairs, Triples, Quads, and RectanglesX-Wing
Learn how exactly two candidate positions create Strong Links, how two aligned links form an X-Wing, and why the X-Wing is your first Fish.
How to find the X-Wing Pattern.Skyscrapers, 2-String Kites, and Strong Links
Learn more about candidate positions and Strong Links, how two links align parallel to form Skyscrapers or align perpendicular to form Kites, and further explore rectangular geometry.
Skyscraper and 2-String KiteThe Empty Rectangle
Using Strong Links and rectangular geometry.
The Empty RectangleXY-Wing (Y-Wing)
Getting back to the Wing Family, this one is not a Fish. The similarity with Wings is the use of Strong Links. Connect three bivalue cells through a pivot and two pincers, then remove their shared candidate from every cell that sees both pincers.
XY-WingXYZ-Wing
Connect two Bivalue Cell pincers through a three-candidate pivot, then remove their shared candidate from every cell that sees the pivot and both pincers.
XYZ-WingUnique Rectangles
Any Standard Sudoku should have exactly one unique solution, and the Unique Rectangle is the edge where a Sudoku puzzle may come the closest to having multiple solutions. This is known as the Deadly Pattern that must be avoided. Change just one corner of the rectangle and the Deadly Pattern is no longer.
Unique RectanglesW-Wing
Connect two matching Bivalue Cells through an outside Strong Link, then remove their other candidate from every cell that sees both wings.
XY-Chain
Connect Bivalue Cells into an alternating relay, then remove the shared endpoint candidate from every cell that sees both ends.
Grouped X-Chain
Follow one candidate through alternating Strong and Weak Links, allowing several confined candidates to act as one logical node.
Alternating Inference Chains - AICs
Following alternating inference to prove where a candidate can no longer be.
Almost Locked Sets and ALS-XZ
Linking Almost Locked Sets with a Restricted Common Candidate to make an elimination.
About Sets Introduction to Sue de Coq
Overlapping Sets and Restricted Common Candidates can also make eliminations.
Simple Coloring Techniques
Simple Coloring follows one candidate digit through conjugate pairs.
Explain My Next Sudoku Move
Get the help not just a hint for how to solve one step towards solving the puzzle, and learn why or how it makes such an impact without giving away the entire solution.
Nishio and Logical Contradiction
Why a documented contradiction proof is not the same as guessing and backtracking.
Put the logic into practice
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