Finned X-Wing
An X-Wing where one row has a third candidate cell — the 'fin'. The elimination shrinks to cells that see both the X-Wing column AND the fin.
Advanced sudoku technique
What it is
A Finned X-Wing is the X-Wing pattern with one twist: in one of the two rows (or columns), the digit has THREE candidate cells instead of two — the standard X-Wing pair plus one extra cell called the 'fin'. The deduction splits in two cases: 1. The X-Wing solves cleanly — the digit takes the standard 2-of-4 corner pattern. Standard X-Wing elimination applies: drop the digit from those two columns in every other row. 2. The fin holds the digit instead — the X-Wing breaks, but the fin's cell IS the digit, which forces eliminations in the fin's row, column, and box. Combining the two cases, the eliminations that hold in BOTH are the only safe ones. Those are cells in the X-Wing's two columns that ALSO share a unit with the fin (typically the fin's box).
When to use it
When you spot a near-X-Wing pattern with one extra cell, usually in the same box as one of the X-Wing corners.
Worked example
Digit 7 in row 2 fits R2C3 and R2C7. In row 6 it fits R6C3, R6C7, and R6C5 (R6C5 is the fin, in box 5 along with R6C7). Standard X-Wing would eliminate 7 from columns 3 and 7 in other rows; with the fin, the elimination shrinks to cells in those columns that see R6C5 too. Cells in box 5 that touch column 3 or column 7 drop 7.
Try it
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