Lesson:Swordfish
Swordfish is the three-row extension of the X-Wing.
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💾 Saved locallyAbout This Lesson
Swordfish is the three-row (or three-column) extension of the X-Wing technique. While X-Wing uses exactly two rows and two columns, Swordfish uses exactly three rows and three columns — and the same core logic applies: a candidate digit's positions across three rows are confined to the same three columns, enabling elimination of that digit from the rest of those columns.
Swordfish is rarer than X-Wing and harder to spot, but it is an essential tool for solving Hard and Expert puzzles. The name was coined in the Sudoku community and has no particular logical significance — it simply helps distinguish it from X-Wing and Jellyfish in conversation and software output.
Understanding Swordfish deepens your grasp of fish logic and prepares you for Jellyfish and the broader framework of Alternating Inference Chains. The jump from X-Wing to Swordfish is primarily one of pattern size — the underlying logic remains identical.
How It Works — Step by Step
Step 1 — Find rows with the digit in 2–3 columns
Identify all rows where your chosen digit appears in exactly 2 or 3 columns. Rows with more than 3 positions cannot be base rows in a Swordfish.
Step 2 — Find three rows whose column positions share exactly three columns
From the candidate base rows, select three such that the union of their candidate column positions is exactly three. If it is more than three, no Swordfish exists for those three rows with that digit.
Step 3 — Verify the Swordfish
Confirm: three base rows, up to three cover columns, all candidate cells in the base rows are within the cover columns, and the cover columns form a set of exactly three.
Step 4 — Eliminate from the cover columns
In the three cover columns, eliminate the digit from every cell not in one of the three base rows.
When to Use This Technique
After X-Wing has been checked and found to not apply. Swordfish is your next fish technique to try when a digit still has too many candidates after X-Wing analysis.
Worked Examples
Digit 2 in Row 1 appears in columns {4, 8}. In Row 5, columns {4, 7}. In Row 9, columns {7, 8}. Union = {4, 7, 8} — exactly three columns. Swordfish confirmed with base rows {1, 5, 9} and cover columns {4, 7, 8}. Eliminate digit 2 from all cells in columns 4, 7, 8 that are not in rows 1, 5, or 9.
Frequently Asked Questions
Can a Swordfish base row have exactly 3 candidates?
Yes. Each base row can have 2 or 3 candidate positions for the digit (but always within the three cover columns). A base row with 3 positions is still valid as long as all three fall within the cover column set.
Is Swordfish more powerful than X-Wing?
Not inherently — it eliminates from three columns instead of two, potentially removing more candidates, but it also appears less frequently. X-Wing should always be checked first.
Related Lessons
Ready to test your knowledge? Try applying this technique in our Hard Sudoku challenges, Expert Sudoku puzzles or explore X-Wing technique guide. Keep training to improve your solve times and master the grid!
Ready to Practice?
Apply this technique on a real puzzle from our daily or practice modes.