📚 The 7 sudoku techniques (easy to expert)
By Miguel Arechaga
Sudoku is learned in layers: each new technique unlocks one more difficulty level. This is the complete ladder. Master the first 4 and you will solve almost any newspaper sudoku; with all 7, almost any sudoku, full stop.
1. Cross-hatching
The technique we all start with: pick a number and mentally cross out the rows and columns where it already appears. Where the crossings leave a single free cell in a box, that is the spot. It fully solves easy levels.
2. Naked single
A cell where, because of its neighbours, only one number fits. Easy to spot with pencil marks: it is the cell with a single candidate.
3. Hidden single
A number that, within a row, column or box, can only go in one cell, even if that cell has several candidates. The bread and butter of medium level.
4. Naked pairs and triples
Two cells of a unit sharing the same two candidates reserve those numbers: erase them from the rest of the unit. Same with three cells and three candidates. This is the first technique that does not place numbers directly but cleans candidates, and that cleanup unlocks everything.
5. Hidden pairs
The harder sibling: two numbers that only appear as candidates in the same two cells of a unit (even if those cells hold more marks). Those two cells belong to them: erase their other candidates.
6. Pointing and claiming candidates
When a number, inside a box, only fits in one row, it can be removed from that row outside the box (pointing). And the reverse: if in a row a number only fits inside one box, remove it from the rest of that box (claiming). This takes you into hard territory.
7. X-Wing
The jewel of the classic techniques. If a number appears as a candidate in exactly two columns of two rows, forming a rectangle, then it must occupy two opposite corners, and you can erase it from the rest of those two columns. The first real X-Wing you spot, you will celebrate.
What next?
There are even more advanced techniques (Swordfish, XY-Wing, colouring chains...), but these 7 solve the vast majority of published sudokus. To see them applied to a real case, continue with how to solve a hard sudoku step by step.
Which technique each level requires
A practical way to measure progress is knowing which techniques each level demands. At MundoSudoku levels are defined by starting clues, and the mapping is quite faithful:
- Easy (40 clues): cross-hatching and naked singles are enough. Ideal for automating how you read the board.
- Medium (32 clues): hidden singles come in and notes become worthwhile. This is where candidate discipline is forged.
- Hard (27 clues): naked pairs and pointing candidates become essential. Without candidate cleanup, you stall.
- Expert (23 clues): hidden pairs and, on the toughest boards, X-Wing. Notes stop being optional: they are the map.
A quick example of each family
Hidden single in practice: imagine the top-left box has three free cells for a 5, but two of them sit in rows that already contain a 5. The third cell is its home, even if it has other candidates pencilled in. The key: don't ask "what fits in this cell" but "where does this number fit".
Naked pair in practice: if two cells in a row hold exactly the candidates 2 and 7, those numbers already have owners. Any other 2 or 7 pencilled in that row gets erased. That erasure often turns a neighbouring cell into a naked single — the domino effect we are after.
Pointing in practice: inside a box, the 4 only fits in two cells and both share the same column. You don't know which of the two it is, but it doesn't matter: no other 4 in that column, outside the box, can exist. Erase them.
The 3 mistakes that stall your progress
First: guessing. Placing a number "to see what happens" trains no logic, and a late contradiction wrecks the game. Second: incomplete notes. Pencilling candidates in only some cells is worse than none, because you trust an incomplete map. Third: not cleaning up after each placement. Every placed number invalidates neighbouring candidates; skip the cleanup and your notes lie. We wrote a whole article on the most common sudoku mistakes.
A suggested training plan
The best routine we know: one daily sudoku (the level rotates through the week, so you touch every difficulty), and whenever a new technique resists you, drop one level and hunt for that exact pattern on purpose all game long. You can also print level packs as PDF to train on paper, where manual notes force you to stay organised.
Frequently asked questions
In what order should I learn the techniques?
In the order of this article: cross-hatching, singles, naked pairs, hidden pairs, pointing/claiming and X-Wing. Each builds on the previous one and the difficulty ramp stays gradual.
When should I start using notes?
As soon as you play medium level. Easy boards are solved by sight, but from 32 clues onwards notes are the only reliable way to apply pairs and eliminations.
Does X-Wing appear in every hard sudoku?
No. Most hard boards fall to pairs and eliminations; X-Wing shows up more often at expert level. But when it appears, it is usually the only key that unlocks the game.
