Swordfish
A rare animal.
The Swordfish is the three-row extension of the X-Wing, and one of the most striking patterns in advanced Sudoku when it appears. It occurs when a digit appears as a candidate in two or three cells across each of three different rows, and every one of those candidate cells falls within the same three columns.
Because the digit must appear somewhere in each of the three rows, and its only legal cells across all three rows are confined to those three columns, the digit is locked into that 3×3 intersection regardless of the exact arrangement each row settles on. That lock lets you eliminate the digit from every other cell in those three columns, outside the three rows that define the pattern. The same logic runs column-first, trading rows for columns. Swordfish patterns are rare and easy to miss, but when the full pattern won't form, a Y-Wing can sometimes deliver the single elimination needed instead.
How It Works
The Swordfish applies when a digit D appears as a candidate in two or three cells across each of three different rows, and all those candidates collectively fall within exactly three columns.
- Row R1 contains D only in some subset of columns
{C1, C2, C3}. - Row R2 contains D only in some subset of
{C1, C2, C3}. - Row R3 contains D only in some subset of
{C1, C2, C3}.
Because D must appear in each of the three rows, and it can only land in those three columns, D is locked into that 3×3 column intersection. It cannot appear in any other cell in columns C1, C2, or C3.
- Find a digit D that appears in two or three cells across exactly three rows.
- Confirm that all those cells fall within exactly three columns.
- Eliminate D from all other cells in those three columns.
The same logic works column-first: find three columns where D appears only within the same three rows, then eliminate from those rows.
Example
Below board presents an example of a swordfish pattern on digit 6, locked into columns 2, 5, and 8 across rows 2, 5, and 8.
Row 2 keeps 6 to columns 2 and 5. Row 5 keeps it to columns 5 and 8. Row 8 keeps it to columns 2 and 8. Between the three rows, 6 never strays outside columns 2, 5, and 8 — so wherever it lands in each row, those three columns absorb all three placements. Every other 6 in columns 2, 5, and 8 can be eliminated.
On Evil Boards
The Swordfish is rare, difficult to see, and devastating when found. Its high yield makes it very useful in all phases of the game. If the pattern won't fully form, a y-wing can sometimes deliver the single elimination that breaks the deadlock instead.