Click cell + number: Place number | Shift + number: Toggle candidate
The simplest technique. When a cell has only one possible number that can go in it (because all other numbers 1-9 already appear in the same row, column, or 3×3 box), that's a naked single. It's "naked" because the single candidate is obvious.
Example: If a cell's row contains 1,2,3,4,5,6,7,8 and only 9 is missing, that cell must be 9.
When a number can only go in one specific cell within a row, column, or box (even though that cell might have other candidates), it's a hidden single. It's "hidden" because you have to look at where the number can go, not just at what can go in each cell.
Example: In a row, the number 5 can only fit in the third cell, even though that cell could also hold 3 or 7. The 5 must go there.
When all possible positions for a number within a box are confined to a single row or column, that number can be eliminated from the rest of that row or column outside the box. This "locks" the candidate to that intersection.
Example: If the number 7 in box 1 can only go in row 2, then 7 can be removed from all other cells in row 2 that aren't in box 1.
When two cells in the same row, column, or box can only contain the same two numbers, those two numbers can be eliminated from all other cells in that unit. They form an exclusive pair.
Example: If two cells in a row can only be {2,5}, then 2 and 5 cannot appear anywhere else in that row.
Similar to naked pairs, but with three cells that collectively can only contain three specific numbers. Each cell doesn't need all three candidates, but together they cover only three numbers.
Example: Three cells with candidates {1,2}, {2,3}, and {1,3} form a naked triple of {1,2,3}.
When two (or three) numbers can only appear in two (or three) specific cells within a unit, those cells can only contain those numbers, eliminating all other candidates from those cells.
Example: If numbers 4 and 6 can only go in cells A and B within a row, then A and B can only contain 4 or 6.
An advanced pattern where a number appears in only two cells in two different rows, and these cells align in the same two columns. This creates an "X" pattern, allowing elimination of that number from other cells in those columns.
Example: If 3 appears only in columns 2 and 7 of rows 1 and 5, forming an X, then 3 can be eliminated from all other cells in columns 2 and 7.
An extension of X-Wing to three rows and three columns. When a number appears in only 2-3 cells in three different rows, and these align with three columns, eliminations can be made.
A pattern involving three cells where logical deduction based on their candidates can eliminate a number from cells that can "see" specific parts of the pattern.
Example: Three cells with {X,Y}, {Y,Z}, and {X,Z} form a wing pattern that can eliminate Z from cells seeing both the XZ and YZ cells.
Easy: Can be solved using only Naked Singles and Hidden Singles. Good for beginners learning pattern recognition.
Medium: Requires Locked Candidates and possibly Naked Pairs. Forces you to look at interactions between boxes and rows/columns.
Hard: Needs advanced techniques like X-Wing, Hidden Pairs/Triples, or XY-Wing. Requires tracking multiple possibilities and complex pattern recognition.
Expert: May require techniques beyond the above, including forcing chains or trial and error. For serious puzzle enthusiasts.
Ultra Hard: Minimal clues (often 17-25). These puzzles push the limits of logical deduction and may require:
• Multiple advanced techniques in combination
• Forcing chains (if X then Y must follow...)
• Trial and error with careful backtracking
• Extreme patience and persistence
These are among the hardest Sudoku puzzles possible while still having a unique solution.