Swordfish in Sudoku: close three lines inside three columns
Follow one digit across three rows. Their candidate positions may form different pairs, but the combined positions must stay inside exactly three columns.
Definition
A Swordfish occurs when three rows confine one digit to exactly three columns, or three columns confine it to three rows; that digit can be removed elsewhere in the crossing lines.
When to use it and what to verify
When to look for it
After checking X-Wing, map one digit in rows that each contain two or three candidate positions.
Required condition
Three base rows must confine every candidate for the digit to exactly three cover columns. A column-based Swordfish reverses the orientation.
Common near miss
If the candidates spread across four cover columns, the three-line reservation is not closed and no basic Swordfish elimination follows.
Rows 2, 5, and 8 keep candidate 4 inside columns 2, 6, and 8.
- 1. Map the three starting rowsRow 2 uses columns 2 and 6, row 5 uses columns 2 and 8, and row 8 uses columns 6 and 8.
- 2. Count the cover columnsAll six pattern candidates lie inside three columns. The three rows need three 4s, so the three columns receive them.
- 3. Remove 4 outside the pattern rowsRemove candidate 4 from columns 2, 6, and 8 outside rows 2, 5, and 8. If the rows spread into a fourth column, there is no Swordfish.
The three base rows must place three 4s within columns 2, 6, and 8. Those columns are fully reserved by the pattern, so candidate 4 is impossible elsewhere in them.
A reference verified by Doku.su
Doku.su checks every placement or elimination in this example with its deterministic Sudoku engine. Illustrative candidates are labelled when they do not represent a complete puzzle state.
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