Sudoku

How to play sudoku here

1. Choose a Difficulty (Easy, Medium, Hard or Expert) and click New grid. A fresh puzzle is generated in your browser in a fraction of a second.
2. Select a cell by clicking or tapping it, or with the arrow keys. The row, the column and the 3 × 3 box of the selected cell are shaded, and cells holding the same number are highlighted.
3. Enter a number with the number buttons or with the keys 1 to 9. Erase (or Backspace) clears the cell.
4. Turn on Notes (or press N) to write small pencil marks instead of answers. Placing an answer automatically removes that number from the notes in the same row, column and box.
5. Check shows the cells that do not match the solution in red. Hint fills in the selected cell, or, if it is already correct, fixes a wrong cell or reveals an empty one. Undo (or Ctrl+Z) goes back one step at a time.
6. Print opens your browser’s print dialog with only the grid on the page: handy for a pen-and-paper puzzle on the train.

The timer runs while the page is visible. Your current game is saved in this browser, so you can close the tab and pick it up later. Your best time for each level is kept when you finish without hints.

The rules

A sudoku grid has 81 cells in nine rows, nine columns and nine 3 × 3 boxes. Fill every empty cell with a digit from 1 to 9 so that each row, each column and each box contains every digit exactly once. The digits in bold are given and cannot be changed. If you place a digit that already appears in the same row, column or box, both cells turn red straight away.

How the grids are made

Each puzzle starts from a complete, randomly filled grid. Cells are then removed in symmetric pairs, and after every removal a solver checks that the puzzle still has exactly one solution. If a removal would allow a second solution, the cell is put back. This is why you never need to guess.

• Easy (about 37–38 givens) and Medium (about 30–31 givens) can always be solved with the two basic techniques below.
• Hard (about 26–30 givens) needs more than those basic techniques.
• Expert (about 23–24 givens) removes as many cells as possible while keeping a single solution, so long chains of deduction are needed.

Solving techniques

• Naked single: a cell where only one digit is still possible, because the other eight already appear in its row, column or box.
• Hidden single: a digit that can go in only one cell of a row, column or box, even if that cell has other candidates.
• Pairs and triples: two cells in a unit that can only hold the same two digits “lock” those digits; remove them from the other cells of the unit.
• Pointing: when all the candidates for a digit in a box lie in one row or column, that digit can be removed from the rest of that row or column.

Notes are the key to Hard and Expert grids: write every candidate, then remove them as you go.

A short history

Number-placement puzzles appeared in French newspapers at the end of the 19th century. The modern form was published in 1979 in the American magazine Dell Pencil Puzzles and Word Games as “Number Place”, probably designed by Howard Garns. The Japanese publisher Nikoli popularised it in the 1980s under the name sudoku (“the digits must be single”), and it swept through newspapers around the world in 2004–2005.