Finned Swordfish
A Finned Swordfish is a Swordfish with extra candidates: a digit has candidates in three rows that fall into three columns, except for one or more cells, the fins, which lie together in one box. Either a fin holds the digit, or no fin does and the plain Swordfish applies. In both cases the digit cannot sit in the cells of the three cover columns that lie in the fin's box, outside the base rows, so it is removed from them. Rows and columns can be swapped. It is an Expert technique in Bare Sudoku.
When to use it
When a Swordfish almost appears: three rows whose candidates for a digit fall into three columns apart from a cell or two in one box. Finned Swordfish are less common than Finned X-Wings; look for them when a digit is spread thinly over several rows. They need complete and accurate notes for that digit.
How to spot it
Pick a digit and list the rows with two to four candidate cells for it. Look for three rows and three columns that cover all of their candidates but one or two; the leftover cells are the fins, and they must lie in one box. Then check the cells of the three columns inside the fin's box, outside the three rows: any that hold the digit lose it. Repeat with columns as the base.
Example
| c1 | c2 | c3 | c4 | c5 | c6 | c7 | c8 | c9 | |
|---|---|---|---|---|---|---|---|---|---|
| r1 | 1 | 458 | 7 | 6 | 3 | 28 | 9 | 2458 | 2458 |
| r2 | 468 | 468 | 26 | 5 | 128 | 9 | 148 | 3 | 7 |
| r3 | 358 | 9 | 23 | 7 | 4 | 128 | 6 | 258 | 1 |
| r4 | 46 | 3 | 16 | 1248 | 7 | 1268 | 5 | 9 | 148 |
| r5 | 9 | 1467 | 5 | 148 | 18 | 168 | 2 | 478 | 3 |
| r6 | 2 | 147 | 8 | 3 | 9 | 5 | 147 | 6 | 14 |
| r7 | 358 | 58 | 9 | 28 | 6 | 7 | 348 | 1 | 2458 |
| r8 | 7 | 2 | 13 | 9 | 158 | 4 | 38 | 58 | 6 |
| r9 | 568 | 1568 | 4 | 128 | 1258 | 3 | 78 | 2578 | 9 |
Bold digits are given, blue digits were placed while solving, small digits are candidates. Highlighted cells form the pattern, crossed-out candidates are removed, and a circled candidate is the digit to place.
- Digit 2 has candidates in rows 1, 4, 7 only in columns 4, 6, 9, apart from one extra cell, r1c8, in box 3. Without r1c8 the pattern would be a Swordfish.
- If r1c8 is not 2, the Swordfish holds: the three rows place their 2 in the three columns, one each, and no other cell of those columns can be 2.
- If r1c8 is 2, no other cell of box 3 can be 2.
- A cell of columns 4, 6, 9 that lies in box 3, outside rows 1, 4, 7, is ruled out either way. Remove 2 from r3c9.
Common mistakes
- Eliminating in the whole of the three columns. With fins, only the cover cells inside the fin's box can be cleared.
- Using fins spread over two boxes. All fins must see the eliminated cells, so they must share one box.
- Counting a cell outside the base rows as a fin. Fins lie in the base rows; extra candidates elsewhere in the cover columns are cells to eliminate, not fins.
In Bare Sudoku
Needed from level: Expert
In the game, the hint says: Finned Swordfish
Frequently asked questions
What is a Finned Swordfish in Sudoku?
A Swordfish whose base rows hold extra candidates, the fins, all inside one box. Either a fin is the digit, or the Swordfish is valid; both cases rule out the digit in the cells of the cover columns that lie in the fin's box, so it is removed from them.
How many fins can a Finned Swordfish have?
Bare Sudoku accepts any number, provided they all lie in one box; one or two fins are the norm. Fins in different boxes would leave no cell that sees them all.
Is there a Finned Jellyfish?
Yes: the same idea with four rows and four columns. Bare Sudoku finds Finned Jellyfish as well, but they are rare, because a digit seldom spreads over four rows without a smaller pattern appearing first.
How is a Finned Swordfish different from a Finned X-Wing?
Only in size: a Finned X-Wing uses two rows and two columns, a Finned Swordfish three of each. The fin rule and the eliminations work the same way, inside the fin's box.
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