Finned Swordfish
A Swordfish with an in-box fin — and its Sashimi degenerate form.
A Swordfish restricts a single digit to three rows (or three columns), where its candidates all fall inside just three shared columns (or rows), letting you erase that digit from everything else in those cover lines. The finned swordfish sudoku technique handles the near-miss version of that pattern: the fish is almost perfect except for one or more stray candidates — the fin — sitting in one of the base lines. Because the fin might turn out to hold the digit, only eliminations that also see the fin still hold. That makes a finned Swordfish cut fewer candidates than a clean one, but it turns up on the grid far more often.
Worked example
Working the digit 1, rows 1, 2 and 5 confine their candidate 1s almost entirely to columns 1, 4 and 7 — the skeleton of a Swordfish. Row 5 spoils the exactness with an extra 1 at R5C8, which becomes the fin. That fin sits in the box spanning rows 4-6, columns 7-9, the same box that holds the column-7 cover cell. A clean Swordfish would strip the 1 from every other cell in columns 1, 4 and 7; here only R6C7 survives the test, because it shares that box and therefore sees the fin. So R6C7 loses its candidate 1.
When to use it
Reach for a finned Swordfish once basic fish, singles and locked candidates have stalled, and a digit is nearly pinned to three lines but one box carries a stray candidate too many. Scan each digit's candidate map for three rows (or columns) that share the same three columns (or rows), then check whether the only intruder sits in a box that also contains a cover cell. If it does, keep just the eliminations that can see that fin.
How to use it
- Find the almost Swordfish. Find three rows (or columns) that nearly confine a digit to three lines, save for a fin of one or two extra spots that all sit in a single box.
- Handle the fin (and Sashimi). If the fin is empty the Swordfish is real; if it holds the digit, the digit lands in the fin’s box. Sashimi is the same idea when the fin props up an otherwise-incomplete line.
- Cut only where both apply. Remove the digit only from cells the basic Swordfish would clear that also see every fin cell — typically the cover cell sharing the fin’s box.
Frequently asked questions
What is a Finned Swordfish in Sudoku?
A Finned Swordfish is a three-line, single-digit fish that would be a perfect Swordfish were it not for one or more extra candidates — the fin — in one of its base lines. The clean fish would eliminate the digit across all three cover columns or rows, but the fin narrows those eliminations to cells that also see it. In the example on this page, working digit 1, the fin at R5C8 limits the whole pattern to a single cut at R6C7.
How do I spot a Finned Swordfish on the grid?
Look for three rows (or columns) that almost lock a digit into three shared columns (or rows), with the pattern broken only by extra candidates crowded into one box. Confirm that box also holds one of the cover cells. Here rows 1, 2 and 5 nearly fish the digit 1 in columns 1, 4 and 7, and the fin at R5C8 shares the rows 4-6 / columns 7-9 box with the column-7 cover cell, so the surviving elimination is the cell that also sees the fin — R6C7.
How is a Finned Swordfish different from a plain Swordfish?
A plain Swordfish is an exact fish: the digit appears only where the three base lines meet the three cover lines, with no stray candidates, so every off-fish cell in those cover lines loses the digit. A Finned Swordfish adds a fin — the extra candidate that breaks that exactness — so it eliminates far fewer cells, only those that also see the fin. On this board a clean Swordfish would have cleared the 1 from all of columns 1, 4 and 7, but the fin at R5C8 leaves just R6C7.