Sudoku strategies
The techniques our solver uses to grade every puzzle, from the first hidden single to the Jellyfish, in order of difficulty.
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Sudoku needs no maths and no guessing, just a handful of logical patterns. The guides below explain each one with a diagram taken from a real puzzle, a step-by-step worked example and three practice puzzles that cannot be solved without that technique. They are in the order we recommend learning them, which is also the order our solver tries them.
Every technique our solver uses
| Technique | Level | In one line |
|---|---|---|
| Hidden single | Easy | A digit with only one possible cell in a row, column or box. |
| Naked single | Easy | A cell with only one possible digit. |
| Pointing pair | Medium | A digit locked to one line inside a box clears it from the rest of the line. |
| Box/line reduction | Medium | A digit locked to one box inside a line clears it from the rest of the box. |
| Naked pair | Medium | Two cells with the same two candidates use up those digits in their unit. |
| Hidden pair | Medium | Two digits confined to the same two cells remove every other candidate there. |
| Naked triple | Hard | Three cells whose candidates are only three digits between them. |
| Hidden triple | Hard | Three digits confined to the same three cells of a unit. |
| X-Wing | Hard | A digit confined to the same two columns in two rows (or vice versa). |
| XY-Wing | Hard | A two-candidate pivot and two pincers that force a shared digit. |
| Swordfish | Expert | The X-Wing idea over three rows and three columns. |
| XYZ-Wing | Expert | An XY-Wing whose pivot has three candidates. |
| Simple coloring | Expert | Two-colouring the strong links of one digit to find contradictions. |
| Unique rectangle (type 1) | Expert | Avoiding a deadly pattern that would give two solutions. |
| Jellyfish | Expert | The X-Wing idea over four rows and four columns. |
Guides for the techniques without a link are not written yet. The solver already explains every one of them step by step on your own puzzle.
How difficulty levels relate to techniques
Every NineByNine puzzle is graded by the hardest technique it needs. An easy puzzle needs only singles. Medium adds pointing pairs, box/line reduction and pairs; hard adds triples, the X-Wing and the XY-Wing; expert adds the Swordfish, Jellyfish, XYZ-Wing, unique rectangles and simple coloring. Nothing on the site ever needs chains or trial and error, so if you know the techniques for a level, you can always find the next step.
A good way to learn is to read one guide, then play its practice puzzles with the hint button handy. The first press of a hint names the technique and where to look, which is exactly the skill these guides build.
Updated · Written by the NineByNine team