Sudoku

How to play sudoku

The grid has 81 squares split into nine rows, nine columns and nine 3×3 boxes. Fill every empty square with a digit from 1 to 9 so that each row, each column and each box contains every digit exactly once. The numbers you start with are fixed. Arithmetic plays no part: the digits are just nine different symbols.

Controls

• Select a square by clicking or tapping it, or move with the arrow keys.
• Enter a digit with the number pad or your keyboard. Press the same digit again, Backspace or Delete to clear it.
• Notes (or the N key) switches to pencil marks, small candidates you can jot in a square while you think.
• Hint fills one correct square, the selected one if it is empty or wrong. Undo steps back as far as you like (Ctrl+Z works too).
• Show mistakes marks digits that do not match the solution. With it off, only clashes in a row, column or box are marked.

What the levels mean

Each puzzle is graded by the hardest technique it needs, plus its clue count.

• Easy (36 to 40 clues): naked and hidden singles are enough. A good warm-up.
• Medium (30 to 33 clues): you will need pointing pairs, box-line reductions and naked or hidden pairs.
• Hard (26 to 31 clues): adds triples and the X-wing pattern.
• Expert (28 clues or fewer): needs advanced patterns such as the swordfish or XY-wing, and often chains of reasoning beyond that.

The daily sudoku is easier early in the week and harder on weekends, and it is the same grid for every player that day.

Strategy tips

Start with singles

A naked single is a square where only one digit is still possible. A hidden single is a digit that can go in only one square of a row, column or box. Scan each box for the digits it is missing and look for the one spot each could take. Most of an easy puzzle is solved this way.

Use notes once singles run out

Write every possible digit in the empty squares of a box or row. Patterns then show up. If two squares in a row both hold only 4 and 7, those two digits belong to those two squares, and you can remove 4 and 7 from the rest of the row. That is a naked pair. If a digit within a box can only sit in one row, it cannot appear in that row outside the box.

Look for the X-wing

When a digit can go in exactly two places in each of two rows, and those places line up in the same two columns, the digit must take one corner in each row. You can then remove it from the rest of those two columns.

Check before you guess

Every puzzle here can be solved by logic alone. If you feel stuck, look for a box or row with few empty squares, or use one hint and continue.

A little history

The puzzle in its modern form first appeared in a US puzzle magazine in 1979 under the name Number Place. It took the name sudoku in Japan in the 1980s and became a worldwide craze in 2005. Mathematicians have shown that a sudoku with a unique solution needs at least 17 clues. If you enjoy adding cages to the logic, try Killer Sudoku, or switch to pictures with a Nonogram.