Start with naked and hidden singles
A naked single appears when one empty cell has only one legal digit after its row, column, and box are checked. It is the most direct placement in Sudoku. Recompute nearby candidates after every single because one placement can expose several more.
A hidden single is found by scanning a region for a digit rather than inspecting one cell. A row may contain several cells with multiple candidates, yet only one of those cells includes 4. The 4 belongs there because the region has no alternative location.
- 01Scan crowded regions for cells with one remaining candidate.
- 02Pick a missing digit and trace where it is blocked in each row, column, or box.
- 03Place the digit only when the cell or region leaves one legal option.
- 04Update peers before searching for a more advanced pattern.
Use locked candidates across boxes and lines
Locked candidates connect a 3×3 box with a row or column. If every possible 7 in a box lies on the same row, the 7 must occur somewhere in that box-row intersection. Therefore 7 can be removed from the rest of that row outside the box. This form is often called pointing.
The reverse relationship is sometimes called claiming. If every possible 7 in a row lies inside one box, other cells in that box cannot contain 7. In both forms, the elimination is justified by confinement, not by choosing which candidate inside the intersection is correct.
Reserve digits with a naked pair
A naked pair occurs when two cells in one row, column, or box contain exactly the same two candidates, such as {2, 9}. Those two digits must occupy those two cells in some order. Other cells in that region can therefore remove 2 and 9.
Both cells must be in the same region, and each must contain only the two pair candidates. Two cells that merely share 2 and 9 among additional candidates do not form a naked pair. After eliminating, look for a newly exposed single rather than trying to resolve the pair immediately.
Recognize an X-Wing without guessing
An X-Wing uses one candidate across two rows and two columns. If candidate 5 appears in exactly the same two columns in each of two rows, the 5s must occupy opposite corners of that rectangle. Other 5 candidates in those two columns can be removed.
The same pattern can be viewed with rows and columns reversed. X-Wing is an elimination technique: it often does not place either corner immediately. Confirm that each base row has exactly two candidates for the digit and that their columns align before removing anything.
Choose the simplest available technique
Advanced patterns are easier to spot after basic deductions have cleaned the grid. Use a consistent order: singles, locked candidates, pairs, then X-Wing. After every elimination, return to singles because the board may have become simple again.
Technique names are useful only when they improve accuracy. State the evidence in plain language: which digit, which region, which candidate cells, and why alternatives are impossible. The Coach turns that evidence into short exercises, while full puzzles show how techniques connect across a solve.
Common questions
Which Sudoku technique should I learn first?
Learn naked singles and hidden singles first. They establish candidate scanning and appear throughout puzzles of every difficulty.
What is the difference between a placement and an elimination?
A placement proves the value of a cell. An elimination proves that one candidate cannot occupy a cell; it may expose a placement later.
Do I need every advanced Sudoku technique?
No. The techniques you need depend on the puzzle set. A small, well-practiced toolkit is more useful than many names you cannot verify on the grid.
Why did my naked pair elimination fail?
Check that both cells share one region and each contains exactly the same two candidates. Extra candidates or cells in different regions invalidate the pair.
Does an X-Wing solve four cells at once?
Usually not. It constrains where one digit can appear and removes that candidate elsewhere. The corner values may remain unresolved until another deduction follows.