Learn Sudoku

32 techniques from first digits to expert chains — each with an interactive lesson and real practice positions.

Every technique page walks you through a real position step by step, shows why the logic holds, and then hands you the board: apply the technique yourself in drill positions taken from generated puzzles. Master a technique by reading its lesson and solving five drills.

Guides

The ideas around the techniques: notation, difficulty, solvers and uniqueness.

Pencil Marks & Notation

Full candidates, Snyder notation — and when to use which.

What Makes a Sudoku Hard?

It's not the number of givens — it's the techniques required.

How Computers Solve Sudoku

Backtracking brute force vs. the technique catalog — and why we use both.

The Uniqueness Argument

Why 'this puzzle has one solution' is itself a solving weapon.

The Basics

Placing digits directly — no pencil marks needed. These four moves solve every easy puzzle.

Last Free Cell

Eight digits placed in a unit — one cell left.

Last Remaining Cell

Cross-hatch a box until one cell survives for a digit.

Naked Single

A cell whose candidates have shrunk to one.

Hidden Single

The digit fits nowhere else in the row or column.

Working with Candidates

Pencil marks become a tool: locked sets and box-line interactions strip candidates and open up the board.

Naked Pair

Two cells, the same two candidates — the rest of the unit lets go.

Hidden Pair

Two digits that only fit in the same two cells.

Naked Triple

Three cells sharing three candidates between them.

Hidden Triple

Three digits confined to three cells — everything else is noise.

Naked Quadruple

Four cells splitting four candidates among themselves.

Hidden Quadruple

Four digits that fit nowhere but the same four cells.

Locked Candidates (Pointing)

A box squeezes its digit into one line.

Locked Candidates (Claiming)

A line claims its digit for one box.

Fish & Wings

Patterns that span several rows and columns. The bread and butter of hard puzzles.

X-Wing

Two rows, two columns, four corners — a rectangle traps the digit.

Y-Wing

A pivot and two pincers squeeze out a third digit.

Swordfish

The X-Wing's big sibling: three rows locked to three columns.

XYZ-Wing

A three-candidate pivot joins both of its wings.

WXYZ-Wing

A pivot and three wings juggle four digits — one must give.

Jellyfish

Four rows, four columns — the largest useful fish.

Finned X-Wing

An X-Wing with a blemish — the fin limits the damage.

Finned Swordfish

A Swordfish with a fin — eliminations shrink to one box.

Finned Jellyfish

The biggest fish, imperfect — the fin still bites in its box.

Advanced Logic

Chains, coloring and uniqueness arguments — the heavy machinery for expert grids.

Skyscraper

Two towers on one base — at least one roof is true.

Two-String Kite

A row string and a column string tied in one box.

Crane

A box link and a line link, joined at the neck.

Empty Rectangle

A box folded onto one row and one column beats a strong link.

W-Wing

Twin cells linked by a strong wire.

Simple Coloring

Paint a digit's strong links in two colors and watch one fail.

X-Chain

One digit, alternating links — the ends cover everything between.

Grouped X-Chain

An X-Chain that moves whole box segments at once.

XY-Chain

A domino run through bivalue cells.

Unique Rectangle

Avoid the deadly pattern — the puzzle has only one solution.

BUG+1

One cell saves the whole board from ambiguity.