Sudoku Technique Visualizer

Fish Patterns

How Fish Logic Works (X-Wing & Swordfish)

A "fish" pattern occurs when a single candidate digit is restricted to $N$ cells across $N$ parallel rows (or columns) such that all these cells align in the same $N$ perpendicular columns (or rows). Because each base row must contain exactly one instance of that digit, the $N$ digits fully occupy the $N$ crossing cover columns, eliminating that candidate from every other cell in those columns.

Bivalue Chains

How Wing Logic Works (XY-Wing)

An XY-Wing uses three cells that each have exactly two candidates. A central “pivot” cell ${X,Y}$ sees two “wing” cells ${X,Z}$ and ${Y,Z}$. Regardless of whether the pivot is $X$ or $Y$, one of the two wings is logically forced to be $Z$. Therefore, any cell in the grid that shares a unit with both wings cannot contain $Z$.

Related Guides

Master Advanced Sudoku Logic

• X-Wing Sudoku Complete Guide
• Swordfish Sudoku Strategy
• Fish, Wings and Alternating Inference Chains
• Sudoku Solver from Photo