Jellyfish

A Jellyfish is a fish pattern involving four rows and four columns. It's the logical extension of X-Wing (2×2) and Swordfish (3×3) to a 4×4 configuration.

How It Works

The Pattern

Look for a digit that:

This creates a 4×4 grid where the digit's placement in those rows completely covers the four columns.

The Logic

If digit 7 appears only in columns 1, 5, 7, 9 across rows 2, 4, 5, and 8:

How to Spot Jellyfish

Single-Digit Focus

The key to spotting fish patterns is focusing on one digit at a time:

Fish Question What to Look For
X-Wing "Where can digit X go in each row?" 2 rows with same 2 columns
Swordfish "Where can digit X go in each row?" 3 rows with at most 3 columns
Jellyfish "Where can digit X go in each row?" 4 rows with at most 4 columns

Step-by-Step Scanning

  1. Pick a digit — Choose any digit 1-9
  2. Scan each row — For that digit, note which columns it can appear in
  3. Find restricted rows — Look for rows where the digit appears in only 2-4 columns
  4. Match columns — Do four rows share the same four columns (or subset)? That's a Jellyfish!
  5. Eliminate — Remove the digit from other cells in those columns

Using Focus Mode

Focus Mode makes Jellyfish much easier to spot:

  1. Tap a digit to highlight all cells containing it
  2. Look at each row — does the digit appear in 2-4 specific columns?
  3. Compare rows — do any four rows share candidates in the same four columns?
  4. If yes, check for eliminations in those columns

Example

Look at digit 7 and ask: where can 7 go in each row?

Row Analysis:

Since rows 2, 4, 5, and 8 "cover" columns 1, 5, 7, and 9, no other cell in those columns can contain 7.

Eliminations: Remove 7 from R3C5, R3C7, R3C9, R6C1, R7C5, and R7C9

puzzle: S9B492b482d069g05804h11041g039x082b7o374p37092t2t045z2g6x3602040h2q1y09012u9e030e0l7x7o620662aa08013m7u1y022m1a0r0e48095w2c062o2l488i54045w012e059g0t7n071u031u0r087o
mode: guided
technique: Jellyfish
initial:
  layers:
    hints: true
steps:
  - text: >
      Use Focus Mode to highlight digit 7. Where can 7 go in each row?
    hint: subtle
    technique: JF
    state:
      focus:
        enabled: true
        digits: [7]

  - text: >
      Rows 2, 4, 5, and 8 all have 7 confined to columns 1, 5, 7, and 9!
    hint: obvious
    technique: JF
    state:
      selection:
        cells: [R2C5, R2C7, R2C9, R4C1, R4C5, R4C9, R5C1, R5C7, R5C9, R8C5, R8C7, R8C9]
      focus:
        enabled: true
        digits: [7]

  - text: >
      Four rows with candidates in only four columns — this is a Jellyfish!
    hint: obvious
    technique: JF
    state:
      selection:
        cells: [R2C5, R2C7, R2C9, R4C1, R4C5, R4C9, R5C1, R5C7, R5C9, R8C5, R8C7, R8C9]
      focus:
        enabled: true
        digits: [7]

  - text: >
      Rows 2, 4, 5, 8 "cover" columns 1, 5, 7, 9. Eliminate 7 from other cells in these columns.
    hint: detailed
    technique: JF
    state:
      selection:
        cells: [R3C5, R3C7, R3C9, R6C1, R7C5, R7C9]
      focus:
        enabled: true
        digits: [7]
settings:
  showCandidates: true
  showControls: true
  showDescription: true
  navigation: numbered

Row vs Column Jellyfish

Row-based:

Column-based:

Both work identically — just swap the dimension.

View in rn-space

A Jellyfish involves 4 rows where a digit is confined to the same 4 columns. In rn-space, this becomes a Naked Quad — 4 cells in a unit sharing 4 candidates.

Jellyfish is one of the hardest fish patterns to spot in the standard grid because it requires tracking 4 rows simultaneously. In rn-space, it appears as a Naked Quad — still not trivial, but a much more familiar pattern that many solvers already know how to find.

puzzle: S9B492b482d069g05804h11041g039x082b7o374p37092t2t045z2g6x3602040h2q1y09012u9e030e0l7x7o620662aa08013m7u1y022m1a0r0e48095w2c062o2l488i54045w012e059g0t7n071u031u0r087o
mode: guided
initial:
  layers:
    hints: false
steps:
  - text: >
      The Jellyfish on digit 7: rows 2, 4, 5, and 8 confine 7 to columns 1, 5, 7, and 9.
    state:
      focus:
        enabled: true
        digits: [7]
      selection:
        cells: [R2C5, R2C7, R2C9, R4C1, R4C5, R4C9, R5C1, R5C7, R5C9, R8C5, R8C7, R8C9]

  - text: >
      In **rn-space**, this becomes a Naked Quad — rows 2, 4, 5, and 8 for digit 7 share candidates {C1, C5, C7, C9}.
    state:
      space: rn
      focus:
        enabled: true
        digits: [1, 5, 7, 9]
        multiDigitMode: 2+
      selection:
        cells: [R2D7, R4D7, R5D7, R8D7]
settings:
  showCandidates: true
  showControls: true
  showDescription: true
  navigation: numbered

The Fish Family

Fish Size Pattern Cells
X-Wing 2 × 2 4 cells
Swordfish 3 × 3 Up to 9 cells
Jellyfish 4 × 4 Up to 16 cells

Each larger fish finds eliminations that smaller fish cannot. However, Jellyfish is rare because:

  1. Four rows must align perfectly
  2. Usually solved by simpler techniques first
  3. Requires many candidates still present

Tips

  1. Start with smaller fish — Find X-Wings and Swordfish first; Jellyfish is rare
  2. Look for concentrated digits — A digit appearing many times in just 4 columns might form a Jellyfish
  3. Use focusing tools — Highlighting a single digit makes the pattern visible
  4. Trust the app — Jellyfish is difficult to spot manually; the hint system will find it

Complexity

Jellyfish is considered an expert-level technique:

Most puzzles never require Jellyfish — if you encounter one, the puzzle is genuinely difficult.

More Puzzles

Related Techniques