How Strategies Work in Sudoku
Sudoku has only three rules — no repeated numbers in any row, column, or 3×3 box. Strategies are the logical tools you use to figure out which number goes where. They do not change the rules. They use the rules to eliminate impossible placements and find forced ones.
Every strategy works by the same principle: if you can prove a number cannot go in a cell, you remove it as a candidate. Eventually, a cell or group is left with only one option — and that is your next placement.
You do not need all strategies at once. Learn them in order, and each one will unlock a harder class of puzzles.
Beginner Strategies (Easy Puzzles)
These two techniques solve most easy puzzles from start to finish. Master them before learning anything else.
1. Scanning
Pick a number (say, 5). Look at every row, column, and box where 5 already appears. In the remaining rows, columns, and boxes, find where 5 can only go in one spot. Place it. Repeat for every number, 1 through 9. This is the foundation of all Sudoku solving. Our tips page covers scanning habits in detail.
2. Naked Singles
A naked single is a cell where only one number is possible. If a cell's row already contains 1, 3, 4, 5, 7, 8, 9, and its column contains 2, then only 6 can go there. As you place more numbers, naked singles appear naturally.
Intermediate Strategies (Medium Puzzles)
When scanning stalls, switch to pencil marks — small candidate numbers written in each empty cell. These techniques work on candidates rather than final placements.
3. Hidden Singles
A hidden single is a candidate that can only go in one cell within a row, column, or box — even if that cell has multiple candidates. This is the single most productive technique in Sudoku. It solves more cells than any other method at every difficulty level.
4. Naked Pairs
A naked pair occurs when two cells in a row, column, or box share the same two candidates and no others. Those two numbers are locked into those two cells, so you can remove them as candidates from all other cells in that group. The same logic extends to naked triples (three cells, three candidates).
5. Pointing Pairs
A pointing pair happens when a candidate within a box is restricted to a single row or column. Since one of those cells must contain the number, you can eliminate it from the rest of that row or column outside the box.
6. Box-Line Reduction
Box-line reduction is the reverse of pointing pairs. When a candidate in a row or column is confined to a single box, you can eliminate it from other cells in that box. Together with pointing pairs, this covers all box-line interactions.
Advanced Strategies (Hard to Expert Puzzles)
These techniques are rarely needed on medium puzzles but become essential as difficulty increases.
7. Hidden Pairs
A hidden pair is two candidates that appear in only two cells within a group — but those cells also contain other candidates. By recognizing the hidden pair, you can strip the extra candidates from those two cells, often revealing further eliminations.
8. X-Wing
An X-Wing forms when a candidate appears in exactly two cells in each of two rows, and those cells align in the same two columns. The candidate must go in two of the four cells (diagonally), so you can eliminate it from all other cells in those columns.
9. Swordfish
A Swordfish extends X-Wing to three rows and three columns. Same logic, larger pattern. This is typically needed only in expert and evil puzzles.
Which Strategies for Which Difficulty?
Easy: Scanning + naked singles. Pencil marks rarely needed.
Medium: Add hidden singles and occasional naked pairs. Pencil marks become useful.
Hard: Add pointing pairs, box-line reduction, and hidden pairs. Pencil marks are essential.
Expert/Evil: Add X-Wing and Swordfish. Full candidate tracking required.
The Learning Path
Follow this order. Each step builds on the previous one:
1. Rules → 2. How to Play → 3. Tips → 4. Naked Singles → 5. Hidden Singles → 6. Pointing Pairs → 7. Box-Line Reduction → 8. Naked Pairs → 9. Hidden Pairs → 10. X-Wing → 11. Swordfish
At each step, practice with the Daily Sudoku or the Deluxe Player at the appropriate difficulty. For offline practice, print a puzzle pack and work through it with a pencil.
If you want to understand why guessing is never necessary, see our guide on solving Sudoku without guessing.
Frequently Asked Questions
What is the best Sudoku strategy for beginners?
Scanning — systematically checking each number across rows, columns, and boxes. This single technique can solve most easy puzzles.
How many strategies do I need?
For easy puzzles, scanning alone is enough. For medium, add hidden singles and naked pairs. Hard adds pointing pairs and box-line reduction. Expert adds X-Wing. Most players are comfortable with 4 to 6 techniques.
What is the difference between strategies and tips?
Strategies are specific logical techniques for finding placements and eliminating candidates. Tips are general habits and approaches that make you more efficient. Both matter.
Do I need to learn all strategies to enjoy Sudoku?
Not at all. Many players happily solve easy and medium puzzles using only scanning and hidden singles. Learn more only if you want to tackle harder puzzles.