Your first game of Sudoku

Set up

The puzzle is a grid of 81 squares, nine rows by nine columns, split into nine boxes of 3×3. Some squares start with a digit printed in them. Those are the givens, and they stay. You fill in the rest so that every row, every column and every box holds the digits 1 to 9 exactly once.

Your first moves

Start where the grid is nearly full. Look for a row, a column or a box with only one empty square. It takes the one digit that is missing. Next look for a digit and where it can go. Pick a digit, say 5, and see where it already sits in the grid. Each 5 rules out the rest of its row, its column and its box. Often there is only one square left in a box where the 5 can go.

When the quick scans find nothing, write the candidates, the digits a square could still hold, small in each empty square as notes. A square left with one candidate takes that digit. Each time you place a digit, cross it out of the notes in its row, column and box, and look again.

How a puzzle ends

It is solved when every square has the right digit. Every puzzle has exactly one solution, and logic always finds it, so you never need to guess. If you’re stuck, a technique you haven’t used yet will take you further.

A short example

A row holds 1, 2, 3, 4, 5, 6, 7 and 9, with one empty square. The digit 8 is the only one missing from the row, so it goes in that square. Then 8 can’t appear again in that square’s column or its box, which cuts down the candidates of every other empty square there.