Platon Sudoku · Sudoku Study

Fish fins · Core Concepts

Fins are extra base candidates outside the selected cover lines. The teal 2 is a fin: it might be true, so ordinary fish eliminations cannot be used throughout the covers.

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Definition

Fins are extra base candidates outside the selected cover lines. The teal 2 is a fin: it might be true, so ordinary fish eliminations cannot be used throughout the covers.

How to find it

First understand the basic fish, then find extras outside the covers. An elimination target must be on a cover, outside the bases, and see every fin.

Common mistake

There may be several fins; the target must see them all. Sharing a corner box helps in this diagram, but proximity alone is insufficient.

X-Wing · Row

Teaching board · local candidate diagram

Observe candidates

Look only at candidate 2, at r2c2, r2c8, r6c2, r6c8, r2c3; the line names can wait.

The teal 2 at r2c3 is outside the covers: a fin; r1c2 is on a cover outside the bases and sees every fin, excluding 2.

Verify the condition

Start from the blue row 2, row 6 and find every 2; each line must eventually contain one 2. These selected starting lines are the bases.

Draw the conclusion

The teal 2 at r2c3 is outside the covers: a fin; r1c2 is on a cover outside the bases and sees every fin, excluding 2. If every fin is false, the ordinary fish excludes 2 at r1c2. If any fin is true, r1c2 sees it and cannot contain 2 either. Both cases exclude only cover targets outside the bases that see every fin.

X-Wing · Column

Teaching board · local candidate diagram

Observe candidates

Look only at candidate 2, at r2c2, r8c2, r2c6, r8c6, r3c2; the line names can wait.

The teal 2 at r3c2 is outside the covers: a fin; r2c1 is on a cover outside the bases and sees every fin, excluding 2.

Verify the condition

Start from the blue column 2, column 6 and find every 2; each line must eventually contain one 2. These selected starting lines are the bases.

Draw the conclusion

The teal 2 at r3c2 is outside the covers: a fin; r2c1 is on a cover outside the bases and sees every fin, excluding 2. If every fin is false, the ordinary fish excludes 2 at r2c1. If any fin is true, r2c1 sees it and cannot contain 2 either. Both cases exclude only cover targets outside the bases that see every fin.

Swordfish · Row

Teaching board · local candidate diagram

Observe candidates

Look only at candidate 2, at r2c2, r2c5, r5c5, r5c8, r8c2, r8c8, r2c3; the line names can wait.

Verify the condition

Start from the blue row 2, row 5, row 8 and find every 2; each line must eventually contain one 2. These selected starting lines are the bases.

Draw the conclusion

Swordfish · Column

Teaching board · local candidate diagram

Observe candidates

Look only at candidate 2, at r2c2, r5c2, r5c5, r8c5, r2c8, r8c8, r3c2; the line names can wait.

Verify the condition

Start from the blue column 2, column 5, column 8 and find every 2; each line must eventually contain one 2. These selected starting lines are the bases.