Sudoku looks deceptively simple: fill a 9×9 grid so every row, column, and 3×3 box contains the digits 1–9 exactly once. But hard puzzles require real technique. Here's everything you need.
Step 1: Scan for singles
Look for rows, columns, or boxes where only one digit is missing — that cell is a "naked single" and fills in immediately. Always do a full scan before trying anything else.
Step 2: Hidden singles
Even when a cell has multiple candidates, check if one digit only appears in one cell across a row, column, or box. If 7 can only go in one place in a row, it goes there — even if that cell still has other candidates.
Step 3: Pencil marks
Write small candidate numbers in each empty cell. This is essential once you move past easy puzzles. Many solvers skip this and then wonder why they're stuck.
Step 4: Naked pairs and triples
If two cells in a row, column, or box each contain only the same two candidates (e.g., both show {3,7}), then those digits must go in those two cells — which means you can eliminate 3 and 7 from every other cell in that row, column, or box.
Step 5: Pointing pairs
If a candidate in a box only appears in one row or column within that box, you can eliminate it from the rest of that row or column outside the box.
Step 6: X-Wing
This technique spans two rows and two columns. If a digit appears in only two cells in each of two different rows, and those cells are in the same two columns, you can eliminate that digit from all other cells in those columns. The same logic applies to columns.
Step 7: Swordfish
An extension of X-Wing across three rows and three columns. Harder to spot, but eliminates candidates in bulk across the grid.
General tips
- Never guess on a proper Sudoku — every puzzle has a unique logical solution.
- If you're stuck, look for boxes with the fewest empty cells first.
- Re-scan after every placement — new singles often appear.
- Colour-coding candidates helps with advanced techniques.
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