Getting started
How to play sudoku.
Sudoku looks like a math puzzle, but it isn't one. You never add or multiply anything: you're just placing nine symbols so none repeats in a row, column or box. You can solve an easy puzzle with three simple techniques, and the first one is plain counting.
Step by step
Your first puzzle, move by move.
You don't need to know any techniques for this. Read the grid, count what's missing, and fill in the cells that can only take one number.
Read one row, column or box
Pick the fullest row you can see, or a box with most of its cells filled. Count which digits from one to nine are already there. The ones missing are the only digits the empty cells of that unit can hold.
Place what only one cell can take
Look at an empty cell and cross off every digit that already appears in its row, its column or its box. If a single digit survives, that cell is that digit. Write it in. That's a single candidate, and it's how most of an easy puzzle gets solved.
Turn the question around
When no cell has a single candidate left, flip it around: pick a digit instead of a cell. Take the digit 7 and ask which cells in one box could still hold it. If only one can, that cell is the 7 even if it can also take other digits. This is a hidden single.
Write the candidates down
Once the easy placements run out, pencil the remaining possibilities into each empty cell. With the marks in, you can read the puzzle at a glance, and every technique after the first two depends on them being complete and correct.
Work where the grid is fullest
A move in a crowded unit is quicker to find and quicker to check than one in an empty one. Finish the easy part of the grid first, and let each number you place make the cells around it easier.
Place, then look around
Every digit you write removes itself from the twenty cells that share its row, column or box. That's often what creates the next single candidate, so re-scan the cells around the one you just filled before moving across the grid.
Don't guess, and don't rush
If you're stuck, what you're missing is a technique, not a lucky number. A right guess teaches you nothing, and a wrong one means undoing half the grid. Take a breath, fill in any marks you skipped, and look again.
The way up
When the easy moves run out.
Harder sudoku doesn't mean bigger numbers or bigger grids. It means a puzzle needs more advanced techniques to crack, and there aren't that many of them.
Easy puzzles: singles alone
Single candidates and hidden singles finish most easy and medium puzzles from the first cell to the last. If you're stuck on one, you've usually missed a single, not a technique.
Harder puzzles: remove, then place
Locked candidates, naked pairs and hidden pairs don't place a digit at all. They take candidates away, and that usually reveals a single a move or two later.
Hard puzzles: three cells at a time
Naked triples and hidden triples work the same way with three cells and three digits. They're rarer, and they reward tidy pencil marks more than a sharp eye.
The hardest: follow one digit
X-Wing is the first technique that asks you to follow a single digit across the whole grid instead of reading one cell or one unit. Past that, the same ideas just get bigger, but these eight will get you through almost any puzzle.
Learning properly
The eight techniques, in order.
Each one has its own page with a worked example and tips for spotting it. They're listed in the order the game teaches them, and each builds on the ones before.
01
Single candidate
The last digit a cell can take, found by counting what its row, column and box already hold.
Beginner02
Hidden single
A digit that fits in only one cell of a unit, even though that cell can take others too.
Beginner03
Locked candidates
A digit trapped in one line inside a box, or one box inside a line, that can be removed elsewhere.
Intermediate04
Naked pair
Two cells in one unit with the same two candidates, which rules those digits out elsewhere.
Intermediate05
Hidden pair
Two digits that can only go in the same two cells, crowding out everything else in them.
Intermediate06
Naked triple
Three cells in one unit whose candidates together come to exactly three digits.
Advanced07
Hidden triple
Three digits that can only go in the same three cells, clearing the rest of the candidates from them.
Advanced08
X-Wing
A digit with two homes in each of two rows, and both rows sharing the same two columns.
Advanced
Learn them by playing.
Sudoku Mori turns every technique on this page into a lesson, with a worked example and three practice puzzles each. It works offline, with no ads and nothing to sign up for.