Quick tips

• Start with the crowded lines. A row or box with four or five numbers often leaves one obvious gap.
• Scan each number. Pick a number, say 3, and look where it can still go in each box.
• Only one place. If a number fits in only one square of a row, column or box, it goes there, even if that square has other candidates.
• Use notes. Pencil in candidates on hard puzzles; placing a number clears it from the notes around it.
• Red means clash. A red number repeats in its row, column or box. Fix it before you build on it.