📚 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.
