Sudoku Solving Techniques & Strategy Guide
Human-logic deductions explained with worked examples, before/after candidate notation, and common traps to avoid.
The Ladder of Sudoku Deduction
Every valid Sudoku puzzle can be solved through pure logic without guessing. In the Ninely mobile game and our daily web warmup, boards are graded not merely by how many empty cells they have, but by the hardest human technique required to advance:
- Easy / Warmup: Full House & Hidden Singles in rows, columns, or 3×3 boxes.
- Medium: Naked Singles requiring careful cross-referencing of peers.
- Hard: Naked Pairs and Hidden Pairs locking candidates.
- Expert: Pointing Pairs and Box-Line Reductions (Locked Candidates).
- Master: Multi-line patterns like X-Wing and Swordfish.
1. Naked Singles
A cell where 8 of the 9 digits are already blocked by existing numbers in its row, column, or 3×3 box. Learn how to scan peers and spot the only possible survivor.
2. Hidden Singles
A number that can only fit in one specific cell within a unit (box, row, or column), even if that cell has multiple candidate options. Learn how to locate them instantly.
3. Locked Candidates
Pointing Pairs and Box-Line Reductions: when candidates for a number are restricted to a single line inside a box, they eliminate that candidate from the rest of the row or column.
4. Naked Pairs
Two cells in the same unit sharing the identical two candidates exclusively. Locking both digits eliminates them from all peer cells, unlocking naked singles.
5. Hidden Pairs
Two digits that appear in only two cells within a unit, buried under other candidate marks. Discard extraneous candidates to lock the pair and crack tough boards.
6. X-Wing
When a candidate appears in only two cells across two parallel rows in the same two columns, it forms a rectangular lock that eliminates candidates across both columns.
7. Naked Triples
Three cells in one unit that together hold only three candidates. Those digits are locked into the three cells and vanish from every other cell in the unit.
8. Swordfish
An X-Wing stretched across three lines: when a candidate in three columns is confined to the same three rows, it is eliminated from the rest of those rows.
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