Swordfish

The 3×3 Fish Pattern for Advanced Elimination

By Minimal Sudoku Team•Last updated: January 27, 2025

The Swordfish is the 3×3 extension of the X-Wing pattern. While X-Wing uses 2 rows and 2 columns, Swordfish uses 3 rows and 3 columns — making it more powerful but harder to spot.

Quick Summary

What: A candidate confined to 3 columns across 3 rows (or vice versa)
Pattern: 6–9 cells forming a 3×3 grid alignment
Result: Eliminate that candidate from other cells in the aligned rows or columns
Difficulty: Hard — requires careful pattern recognition

The Concept

A Swordfish occurs when a candidate appears in at most 3 positions in each of 3 different rows (or columns), and all those positions align within the same 3 columns (or rows).

Unlike X-Wing which always has exactly 4 cells, a Swordfish can have anywhere from 6 to 9 cells — not all positions in the 3×3 grid need to be filled.

Row-Based Swordfish

Find 3 rows where a candidate appears in only 2–3 cells each, all within the same 3 columns.

→ Eliminate from those columns

Column-Based Swordfish

Find 3 columns where a candidate appears in only 2–3 cells each, all within the same 3 rows.

→ Eliminate from those rows

The Golden Rule

🗡️ Swordfish Rule

When a candidate is confined to the same 3 columns across 3 rows (with 2–3 occurrences per row), eliminate that candidate from all other cells in those 3 columns. The same applies with rows and columns swapped.

How to Find a Swordfish

Find rows with limited candidates

Look for rows (or columns) where a specific candidate appears in only 2 or 3 cells. For example, digit 8 appears only in columns 2, 4, and 6 of row 2.

Find two more matching rows

Look for two additional rows where the same candidate also appears in 2–3 cells — and those cells must fall within the same 3 columns as the first row.

Verify the alignment

Confirm all candidate positions across the 3 rows are confined to exactly 3 columns. Each column should have the candidate in at least 2 of the 3 rows.

Eliminate candidates

Remove the candidate from all other cells in those 3 columns (for row-based) or 3 rows (for column-based Swordfish).

Practical Example

Swordfish on Digit 8

Swordfish pattern on digit 8 in columns 2, 4, and 6

In this example, we've found a column-based Swordfish for the digit 8:

Column 2: Digit 8 appears only in rows 2, 6, and 7
Column 4: Digit 8 appears only in rows 2, 6, and 7
Column 6: Digit 8 appears only in rows 2, 6, and 7

All three columns have their 8s confined to the same three rows. This means rows 2, 6, and 7 will each contain exactly one 8 from these columns.

Elimination

Remove 8 as a candidate from all other cells in rows 2, 6, and 7 (outside columns 2, 4, and 6).

Why It Works

The logic extends directly from X-Wing. In each of the 3 columns, the candidate must appear somewhere. Since it can only go in cells within rows 2, 6, or 7, those three rows will "absorb" all three 8s — one per column.

We don't need to know exactly which cell in each column gets the 8. We just know that between them, rows 2, 6, and 7 will claim all three 8s from these columns. Therefore, no other cell in those rows can contain an 8.

Visual representation:

Rows 2, 6, and 7 align with columns 2, 4, and 6 — a Swordfish on 8.

→ Eliminate 8 from other cells in rows 2, 6, and 7.

Not All 9 Positions Required

A valid Swordfish doesn't need all 9 cells filled. As long as each row has the candidate in 2–3 of the defining columns (and vice versa), the pattern works. Six cells is the minimum.

Fish Family Comparison

Pattern	Size	Difficulty
X-Wing	2 rows × 2 columns	🟡 Medium
Swordfish	3 rows × 3 columns	🟠 Hard
Jellyfish	4 rows × 4 columns	🔴 Expert

Each fish pattern follows the same logic — just at different scales. Master X-Wing first, then Swordfish becomes a natural extension.

Detection Tips

Start with X-Wing Candidates

When looking for X-Wings, if you find a candidate appearing in 3 cells in a row instead of 2, don't dismiss it — check if it could be part of a Swordfish instead.

Focus on Rare Candidates

Candidates that appear 8–12 times in the grid are ideal Swordfish targets. Too common (15+) means too many possibilities; too rare (5–6) and X-Wing is more likely.

Look for Incomplete Patterns

Don't search for perfect 3×3 grids. A valid Swordfish often looks "incomplete" with only 6–7 cells. The key is that each row/column has 2–3 candidates within the same 3 columns/rows.

Common Mistake

Make sure all three defining rows (or columns) contribute to the pattern. If one row has candidates in columns outside your suspected Swordfish, the pattern breaks.

Swordfish sits in the middle of the Fish family. Understanding its relationship to X-Wing and Jellyfish helps solidify the underlying logic.