Rules & techniques

Sudoku rules

The three fundamental rules of 9×9 sudoku: each digit 1–9 once per row, column, and 3×3 block.

Last free cell

Fill the last empty cell in a row, column, or 3×3 block with the only digit that hasn't been used yet.

Last cell left

Use a digit's known positions to rule out rows, columns, and blocks until only one cell remains.