Rules

Sudoku rules

Sudoku takes no arithmetic and no guessing. There are exactly three rules, and every puzzle has exactly one solution.

The grid

A Sudoku has 81 cells: nine rows, nine columns and nine boxes of 3 × 3 cells each. Some of the cells are already filled in – these are the givens. Your job is to fill in the rest.

The three rules

1. Every row contains each digit from 1 to 9 exactly once.
2. Every column contains each digit from 1 to 9 exactly once.
3. Every 3 × 3 box contains each digit from 1 to 9 exactly once.

That's all there is. Digits are never added up or compared – they are just symbols. You could play with nine colors instead.

Getting started

Don't hunt for the hardest cell, look for the easiest one. Pick a digit – say the 5 – and go through the boxes that are still missing it. Often the 5s in the neighboring rows and columns rule out so many cells that only one is left.

That is already a real solving technique: a hidden single. Its counterpart is the naked single – a cell where the three rules leave only one possible digit. These two techniques alone will solve any easy Sudoku from start to finish.

Using notes

As soon as a cell has more than one option, notes pay off: small digits in the cell that record what could still go there. Without notes you carry half the puzzle in your head – with them, ruling things out becomes simple bookkeeping.

In neun. you add a note with Shift + digit, without switching modes. A fills in every currently possible candidate at once.

When to guess

Never. A properly built Sudoku can always be solved without guessing – every puzzle here is checked for that when it's generated. If you're stuck, you're missing a technique, not luck.

On to the Sudoku techniques – or straight to a puzzle.