From the first rule to the tricky techniques

How to solve a sudoku: the rules and the techniques

One rule holds the whole puzzle together: every digit from 1 to 9 appears exactly once in every row, every column and every 3×3 box. The rest is a matter of looking. Below you find the rules first, then the seventeen techniques spread over six pages — each technique with an animation on a real board — and finally the button that tells you which step comes next while you play.

The rules of sudoku

A sudoku is a grid of 9×9 squares, divided into nine 3×3 boxes. Some of the digits are already there: those are the givens, and those squares never change. You fill in the rest yourself with the digits 1 through 9, so that each digit appears once in every row, every column and every box.

Every sudoku here has exactly one solution and can be solved without guessing. So there is always a next step you can find with logic. If you guess anyway and put a wrong digit down, Check shows where it goes wrong.

The techniques, from easy to hard

Seventeen techniques, spread over six pages. Each technique comes with an animation on a real board: you see the pattern light up and the candidates disappear, step by step, and you can go back and forth yourself. Start at the top — each page builds on the one before.

Getting started: singles

The basics: one digit still fits, or one place is still free.

Naked single · Hidden single

Step 1 of 6

Intersections

What a box and a row share tells you something about both.

Pointing pair · Box/line reduction

Step 2 of 6

Pairs and subsets

Squares that claim a set of digits together keep them from the rest.

Naked pair · Hidden pair · Naked triple · Hidden triple · Naked quad · Hidden quad

Step 3 of 6

Fish

One digit, two to four rows, and the same columns.

X-wing · Swordfish · Jellyfish

Step 4 of 6

Wings

Three squares that see each other, and one digit that has to give way.

XY-wing · XYZ-wing · W-wing

Step 5 of 6

Coloring and chains

Two colors in turn, and the chain does the rest.

Simple coloring

Step 6 of 6

The Hint button: the step instead of the answer

The game has one Hint button, and it never gives away a digit. It tells you which step can be taken logically right now, in plain words: "in box 4 the 7 can only go in this row". It comes in three small steps you open yourself: first only where to look, then which technique it is and what it means, and then what follows from it here. The squares of that step light up, and you place the digit yourself. That way you learn the technique instead of the answer.

If there is a wrong digit somewhere, the hint says so first: thinking further makes no sense then. Which square is wrong stays a secret; that is what Check is for. And if you play in a room, everyone reads the explanation along, so you learn from it together.

Notes: your candidates in a square

Almost every technique on the pages above works with candidates: the digits that can still go in a square. With notes on, you put them in a square in small print, up to four in the corners, and only then do the patterns really show. Without candidates written down, a naked pair is almost impossible to find.

If you like, the site can keep them up to date for you: automatic fill puts in all the candidates, and automatic cleanup removes a digit from the notes in its row, column and box as soon as it is really placed somewhere. In a room everyone sees the same notes, so you think together on the same board.

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