Advanced Sudoku Strategy
Swordfish Sudoku Technique: Step-by-Step Guide and Examples
Learn how a single candidate can form a three-row and three-column pattern. Swordfish lets you remove candidates across the grid and break through difficult Sudoku puzzles without guessing.
Three Rows, Three Columns
When one candidate is restricted to the same three columns across three rows, it must occupy those intersections.
Choose One Candidate
Swordfish always follows the positions of one candidate number at a time.
Find Three Lines
Look for three rows whose candidate positions are limited to the same three columns, or the reverse.
Remove Candidates
Remove the candidate from other cells in the three intersecting columns or rows.
What Is a Swordfish in Sudoku?
A Swordfish is an advanced Sudoku candidate-elimination pattern involving three rows and three columns.
For one candidate number, suppose the possible positions in three different rows are all restricted to the same three columns. Because each of those rows must eventually place the candidate somewhere within those three columns, the candidate must occupy three of the intersections.
That means the same candidate can be removed from every other unsolved cell in those three columns.
The pattern can also work in the opposite direction: three columns can restrict a candidate to the same three rows.
The Swordfish Rule
One Candidate + Three Rows + Three Columns
If all possible positions for one candidate across three selected rows are restricted to the same three columns, that candidate must occupy those three columns within the selected rows.
Therefore, the same candidate can be removed from other rows in those three columns.
How to Find a Swordfish
Choose One Candidate
Select one number from 1 to 9 and examine every possible position for that candidate.
Find Rows With Few Positions
Look for rows where the selected candidate appears only two or three times.
Compare Three Rows
Check whether the candidate positions across three rows are all contained within the same three columns.
Confirm the Pattern
The three selected rows must collectively use no more than the same three columns for that candidate.
Eliminate From the Columns
Remove the candidate from every other unsolved cell in those three columns outside the selected rows.
Row-Based Swordfish Example
Suppose you are tracking candidate 7.
Candidate 7 Positions
Across rows 2, 5, and 8, candidate 7 appears only in columns 2, 6, and 9.
Remove candidate 7 from every other unsolved cell in columns 2, 6, and 9 outside rows 2, 5, and 8.
Column-Based Swordfish
A Swordfish does not need to start with rows. The same logic can begin with three columns.
Example
Suppose candidate 4 in columns 1, 5, and 8 can appear only in rows 2, 6, and 9.
Those three columns must place candidate 4 within those three rows, so candidate 4 can be removed from other cells in rows 2, 6, and 9 outside columns 1, 5, and 8.
Swordfish Does Not Require Exactly Six Cells
A common misunderstanding is that every row in a Swordfish must contain exactly two positions for the candidate.
That is not required.
The Important Rule
The candidate positions across the three selected rows must all be contained within the same three columns.
One selected row may contain two candidate positions while another contains three.
Why Swordfish Works
Every digit must appear exactly once in each row and exactly once in each column.
If candidate 5 in three rows can only appear in three particular columns, each of the three rows must eventually place its 5 somewhere in those columns.
Those three columns are therefore reserved for the three placements belonging to the selected rows.
The Key Relationship
Three placements are restricted to three intersecting lines. That restriction prevents the same candidate from appearing elsewhere in those lines.
Swordfish vs. X-Wing
If Swordfish feels difficult, study our X-Wing Sudoku guide first. Swordfish uses the same general logic on a larger pattern.
Swordfish vs. Locked Candidates
For a smaller candidate-restriction pattern, read our Locked Candidates Sudoku guide .
When Should You Look for Swordfish?
After Simpler Techniques Stop
Use Swordfish only after checking singles, pairs, Locked Candidates, and X-Wing.
When One Candidate Repeats Sparsely
A candidate with only two or three possible positions in several rows is worth checking.
In Expert Sudoku
Swordfish is most useful in harder puzzles where basic candidate eliminations are not enough.
Before More Complex Chains
Check fish patterns before moving into advanced chains or coloring techniques.
When Is It Not a Swordfish?
❌ Candidate Appears in a Fourth Column
If the candidate positions across the selected rows require four columns, the three-column Swordfish pattern is not valid.
❌ You Are Mixing Candidates
Every part of one Swordfish must track the same candidate number.
❌ Candidate Notes Are Incomplete
Missing a valid candidate position can make an invalid pattern appear to be a Swordfish.
❌ The Selected Lines Do Not Share Three Opposite Lines
Three selected rows must collectively restrict the candidate to three columns, or vice versa.
Common Swordfish Mistakes
❌ Eliminating From the Pattern Rows
For a row-based Swordfish, keep the candidate in the selected rows and remove it from other rows in the three target columns.
❌ Requiring Exactly Two Positions Per Row
A selected row may contain three candidate positions as long as all positions stay within the same three columns.
❌ Confusing Swordfish With X-Wing
X-Wing uses two lines in each direction. Swordfish uses three.
❌ Expecting a Number Placement
Swordfish is mainly an elimination technique. Rescan after making the eliminations.
How to Practise Swordfish
1. Master X-Wing First
Understanding the two-row X-Wing pattern makes the three-row Swordfish structure much easier to recognise.
2. Scan One Candidate at a Time
Choose one digit and map its possible positions across all nine rows.
3. Look for Sparse Rows
Rows containing only two or three possibilities for your candidate are the easiest places to start.
4. Reverse the Search
If no row-based Swordfish appears, repeat the process using columns.
5. Rescan After Eliminations
Removing even one candidate may reveal a single, pair, Locked Candidate, or X-Wing.
Swordfish Practice Checklist
- 1. Are your candidate notes complete?
- 2. Are you tracking only one candidate?
- 3. Did you find three useful rows or columns?
- 4. Are all candidate positions restricted to exactly three opposite lines?
- 5. Did you verify that no selected line contains a position outside the pattern?
- 6. Are you eliminating only the same candidate?
- 7. Are your eliminations outside the three selected base lines?
- 8. Did you rescan the puzzle afterward?
Related Sudoku Learning Pages
🧠 Advanced Sudoku Techniques
Explore advanced Sudoku strategies including subsets, fish patterns, candidate interactions, and chains.
✖️ X-Wing Sudoku
Learn the simpler two-row and two-column fish pattern before mastering Swordfish.
🔒 Locked Candidates Sudoku
Learn how box-line interactions create candidate eliminations.
🕵️ Hidden Pair Sudoku
Find two candidates restricted to the same two cells and remove distracting candidate notes.
Frequently Asked Questions
What is a Swordfish in Sudoku?
A Swordfish is an advanced candidate pattern in which one number is restricted across three rows and three columns, allowing that candidate to be removed from other cells in the affected lines.
How is Swordfish different from X-Wing?
X-Wing uses two rows and two columns, while Swordfish extends the same candidate-restriction principle to three rows and three columns.
Does every Swordfish row need exactly two candidate positions?
No. Each selected row may contain two or three positions as long as all candidate positions across the three rows are contained within the same three columns.
Is Swordfish an advanced Sudoku technique?
Yes. Swordfish is usually considered an advanced strategy and is best learned after techniques such as Locked Candidates, pairs, and X-Wing.
Does Swordfish solve a cell directly?
Usually not. Swordfish removes candidates, which can then reveal singles or other solving patterns.
Practise the Swordfish Technique
Open an Expert Sudoku puzzle, turn on Notes, select one candidate number, and scan the grid for three rows whose possible positions are restricted to the same three columns.