How to play Sudoku

Sudoku is a number puzzle on a grid. You fill every empty cell so that the numbers never repeat. It needs no maths, only patience.

The rules

1. The classic grid has 9 rows, 9 columns and nine 3×3 boxes.
2. Put the digits 1 to 9 in every row, once each.
3. Do the same in every column and every box.
4. Some digits are printed already. Every puzzle has exactly one solution.

Where to start

Pick a digit that is already common on the grid, say 5. Find the rows and columns that hold a 5. The cells they cover cannot take another 5. Often one cell in a box is left: that is where the 5 goes.

Pencil marks

When no cell is certain, write the possible digits small in each empty cell. Cross a digit out when a peer in its row, column or box takes it. A cell with one mark left is solved.

Harder grids

• Naked pairs: two cells in a unit that hold only the same two digits. Remove those digits from the rest of the unit.
• Pointing pairs: a digit in a box lies in one line only. Remove it from the rest of that line.
• X-wing: a digit sits in the same two columns in two rows. Remove it from the rest of those columns.

Other grids

Jigsaw grids swap boxes for irregular shapes. Diagonal grids add the two diagonals. Hyper grids add four extra boxes. 12×12 and 16×16 grids use more digits, with the same rules.

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