Platon Sudoku · Sudoku Study

Fish overview · Core Concepts

Fish techniques examine the positions of one digit. In the simplest X-Wing, two rows can place that digit only in the same two columns, so other rows cannot place it in those columns. The four candidates at the row-column intersections are the rectangle’s four corners.

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Definition

How to find it

Focus on one digit: choose two rows with exactly two positions each, both in the same two columns; then exchange rows and columns.

Common mistake

Check every candidate in the chosen rows, not just four attractive corners. Base and cover lines name these rows and columns; the following concepts explain them.

X-Wing · Row

Teaching board · local candidate diagram

Observe candidates

Look only at candidate 2, at r2c2, r2c8, r6c2, r6c8; the line names can wait.

All 2s in row 2, row 6 lie in column 2, column 8; r1c2, on a cover outside the bases, cannot contain 2. These four candidates form the X-Wing’s four corners.

Verify the condition

Each selected base, row 2, row 6, needs a 2, and all their 2s lie in column 2, column 8. Blue starting lines are called bases.

Draw the conclusion

Amber crossing lines are called covers. The 2 bases need 2 copies of 2. No cover can contain two, so each cover is occupied by a base 2. The target r1c2 lies on a cover outside the bases and loses 2.

X-Wing · Column

Teaching board · local candidate diagram

Observe candidates

Look only at candidate 2, at r2c2, r8c2, r2c6, r8c6; the line names can wait.

All 2s in column 2, column 6 lie in row 2, row 8; r2c1, on a cover outside the bases, cannot contain 2. These four candidates form the X-Wing’s four corners.

Verify the condition

Each selected base, column 2, column 6, needs a 2, and all their 2s lie in row 2, row 8. Blue starting lines are called bases.

Draw the conclusion

Amber crossing lines are called covers. The 2 bases need 2 copies of 2. No cover can contain two, so each cover is occupied by a base 2. The target r2c1 lies on a cover outside the bases and loses 2.