Free tool / Pattern visualizer
Explore advanced solving techniques with color-coded base rows, intersecting cover units, and candidate elimination breakdowns.
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Fish Patterns
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
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 $Z$.
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