Sudoku

About Sudoku

Modern Sudoku was designed by American architect Howard Garns in 1979 under the name "Number Place" and first published in Dell Pencil Puzzles & Word Games magazine. The puzzle was later picked up by Japanese publisher Nikoli in 1984, who gave it the name Sudoku, meaning "single number." It remained a niche favorite in Japan until 2004, when retired judge Wayne Gould introduced it to The Times of London, sparking a worldwide craze. Mathematicians have calculated that there are approximately 6.7 sextillion valid completed Sudoku grids, ensuring every puzzle presents a unique logical challenge. No guessing is ever required in a properly constructed Sudoku; pure deduction and elimination will always lead to the solution.

How to play

1. Fill every cell in the 9x9 grid so that each row, column, and 3x3 box contains digits 1 through 9.
2. Click or tap a cell and type a number to place it.
3. Use pencil notes to track possible candidates for tricky cells.
4. Navigate with arrow keys and use logic, never guessing, to solve the puzzle.

The solving techniques, in the order you need them

Scanning and the naked single. Pick a digit, say 5, and look at every row, column and box that already contains one. Those lines are now closed to another 5. Very often that leaves exactly one cell in some box where a 5 can go. This one technique solves most easy puzzles outright, and it is the only technique that gets faster the more of the grid you fill.

The hidden single. The trap for new players is looking only for cells with one candidate. Just as common is a digit with one home: a box where 7 could go in three cells at first glance, but two of them are blocked by a 7 elsewhere in their row. The cell may still have several candidates of its own, yet the 7 has nowhere else to live. Beginners stall on puzzles that are full of hidden singles because they are searching cells rather than digits. Search both.

Naked pairs and triples. If two cells in the same row both have exactly the candidates 3 and 8, then 3 and 8 occupy those two cells in some order, and you may strike 3 and 8 from every other cell in that row. You have not placed a digit, but you have usually created a single somewhere else. This is the first technique that feels like real deduction rather than looking.

Pointing pairs. If within one box the only places for a 4 all sit in the same column, then the 4 for that box is somewhere in that column, so no other box may use a 4 in it. This is how you break most medium puzzles that appear stuck, and it is the point at which pencil notes stop being optional.

Common mistakes

• Guessing. A properly built Sudoku has exactly one solution reachable by pure logic. A guess that happens to be wrong stays invisible for another twenty placements and then poisons the whole grid, and you cannot tell which of your entries was the lie.
• Not writing candidates down. Beyond easy difficulty, holding candidates in your head is what makes people believe they are bad at Sudoku. They are not; they are running out of working memory.
• Hunting cells only. Half the available deductions come from asking where a digit can go, not what a cell can hold.
• Abandoning a region after one pass. Every digit you place changes the constraints on regions you already scanned. When stuck, rescan the box you filled most recently.

Which difficulty should you play?

Difficulty in Sudoku is not about how many digits are given. It is about which techniques the puzzle forces you to use. An easy puzzle can be finished with scanning alone. Medium puzzles require hidden singles and usually naked pairs. Hard and expert puzzles are constructed so that no simple technique breaks them, and pointing pairs or worse become mandatory. If you are stuck on a puzzle for twenty minutes, the useful question is not "what am I missing" but "which technique have I not tried".

More Sudoku and logic puzzles

Pick your level: Easy Sudoku, Hard Sudoku or Expert Sudoku. For variants that change the rules rather than the difficulty, try Killer Sudoku, which adds cage sums, or Snake Sudoku. For a different kind of deduction entirely, try Picross or Binary Puzzle.