How to read the grids
- Given - clues from the starting puzzle
- Subject - cells the technique focuses on
- Eliminated - candidates this technique removes
- Candidate - pencil marks shown for reference
Naked Single
A naked single is a cell that has only one possible value left after eliminating digits already present in its row, column, and 3x3 box. It is the most fundamental technique in Sudoku - every puzzle solution is built on chains of naked singles.
Read the full guide BasicHidden Single
A hidden single is a digit that can only legally go in one cell within a row, column, or 3x3 box - even though that cell has other candidates. It is the second most common technique and the foundation of cross-hatching.
Read the full guide BasicNaked Pair (and Triple)
When two cells in the same row, column, or box have identical candidate pairs (say, only {3, 8}), those two digits are locked into those two cells. They can be eliminated from every other cell in the same unit.
Read the full guide BasicHidden Pair (and Triple)
When two digits can only appear in the same two cells within a unit - even though those cells have other candidates - the two digits form a hidden pair. All other candidates can be eliminated from those two cells.
Read the full guide IntermediatePointing Pair (Box-Line Reduction)
When a digit can only appear in cells of one row or column within a box, that digit must be placed in that row or column - which means it cannot appear elsewhere in the row or column outside the box.
Read the full guide IntermediateBox-Line Reduction (Claiming)
The inverse of a pointing pair. When a digit can only appear in the part of a row or column that intersects a single 3x3 box, the digit is locked inside that box - eliminate it from the rest of the box.
Read the full guide IntermediateX-Wing
An X-Wing is the simplest fish technique. When a digit appears in exactly two cells of two different rows, and those four cells form a perfect rectangle in two columns, the digit can be eliminated from every other cell of those two columns (and vice versa).
Read the full guide AdvancedSwordfish
A Swordfish is the 3-row/3-column generalization of the X-Wing. If a digit has candidates in at most three cells across three different rows, and those candidates all fall within the same three columns, the digit can be eliminated from those columns outside the three rows.
Read the full guide AdvancedXY-Wing
An XY-Wing uses three bi-value cells - the "pivot" and two "wings" - to eliminate a single digit. If the pivot pencils {X, Y}, one wing pencils {X, Z}, the other wing pencils {Y, Z}, and both wings see the pivot, then any cell seen by BOTH wings cannot contain Z.
Read the full guide AdvancedXYZ-Wing
A natural extension of the XY-Wing. The pivot pencils three candidates {X, Y, Z}, the two wings pencil {X, Z} and {Y, Z}, and both wings see the pivot. Any cell that sees the pivot AND both wings cannot contain Z.
Read the full guide AdvancedUnique Rectangle
A well-formed Sudoku has exactly one solution. The Unique Rectangle technique exploits this: if four cells in two rows, two columns, and two boxes are all about to pencil the same two digits, the puzzle would have two solutions - so one of those candidates must be eliminated.
Read the full guide AdvancedSimple Coloring
When a digit appears in exactly two cells of a unit, those two cells form a "conjugate pair" - one is true, the other is false. Chain conjugate pairs together with two colors and you can deduce contradictions that eliminate the digit from cells that see both colors.
Read the full guideWhich technique do I need for which difficulty?
Different difficulty levels demand different techniques. Use this as a rough guide for what to study next.
| Difficulty | Techniques you need | Notes |
|---|---|---|
| Very Easy / Easy | Naked Single, Hidden Single | Pure scanning. Pencil marks are optional. |
| Medium | Naked Pair, Hidden Pair, Pointing Pair, Box-Line Reduction | Pencil marks become essential. Most "stuck" moments are an unspotted pair. |
| Hard | X-Wing, Naked Triple, Hidden Triple, Coloring | Fish techniques and chain logic come into play. |
| Expert | Swordfish, XY-Wing, XYZ-Wing, Unique Rectangle, deeper coloring | Multiple advanced techniques chained together on a single grid. |
Time to practice
Reading is half the work. Try a puzzle now and look for the techniques you just learned.