How to play sudoku

Sudoku takes a minute to learn: fill the grid so that every row, every column and every box holds the digits 1 to 9 once. Here are the rules, how to find your first digits, and what makes a puzzle hard.

The rules

• The grid has 9 rows, 9 columns and 9 boxes of 3 × 3 cells.
• Each row, each column and each box must hold every digit from 1 to 9, each exactly once.
• Some digits are given at the start. You fill in the rest.
• A good puzzle has exactly one solution, and you never need to guess. Every one in Sudoku Knockout is checked for that.

Your first digits

You never write a digit because it might fit. You write it because it’s the only one that can. The names on the edges help: B7 is row B, column 7. Here are the first three moves of an Easy puzzle, as the game’s hints show them.

The first two are hidden singles: the only place left for a digit in a box. The third is the easiest move of all, the last empty cell of a row. Easy puzzles need nothing else.

When it gets harder: notes

From Medium puzzles on, some cells can only be found by asking which digits they have left. That’s what notes are for: small digits in a cell, for each digit it can still take. A cell with one note left is a naked single. Harder puzzles are all about ruling notes out, with patterns like locked candidates, pairs and the X-Wing.

Tips for your first puzzles

• Start with the boxes, and with the digit you see most often on the board.
• After each digit you place, look again at its row, column and box: it often leads straight to the next.
• Don’t guess. A wrong digit counts as a mistake in Sudoku Knockout, and three end the game.
• Stuck? A hint fills in one cell and shows why it goes there, so you learn the move for next time.

Difficulty levels

A puzzle’s level is the hardest technique it needs. Every level adds a few:

• EasyHidden single
• MediumNaked single
• HardLocked candidates and Naked and hidden pairs
• ExpertX-Wing, Triples and quads, Skyscraper and 2-String Kite and XY-Wing
• MasterSwordfish and Jellyfish, XYZ-Wing, W-Wing and Quads
• ExtremeChains: X-Chain, XY-Chain and AIC

Play with friends

Once you know the rules, sudoku is even better against someone. In Sudoku Knockout you can race friends on the same puzzle, share one board in a battle, or play knockout rounds with up to six players. See how multiplayer sudoku works.