Swordfish

Outcome

You can extend Fish logic to three base and three cover units, including irregular source distributions where the three rows do not look alike.

Intro copy

Swordfish is not “a complicated X-Wing picture.”

It is the same Fish count:

three bases → three covers

The challenge is seeing the cover union when the source pattern is irregular.

Step 1 — Grow X-Wing into Swordfish

Start:

2 bases / 2 covers
→ X-Wing

Add one base row and one cover column:

3 bases / 3 covers
→ Swordfish

Show an irregular source map:

r1 → c1/c8
r4 → c1/c5/c8
r9 → c5/c8

Prompt:

How many distinct cover columns exist?

Correct: 3.

Step 2 — Candidate union by unit

Learner selects the three base rows.

UI displays:

row candidate columns:
{1,8}
{1,5,8}
{5,8}

cover union:
{1,5,8}

Prompt:

Three bases. Three covers. What can be removed?

Learner removes target digit from the cover columns outside the bases.

Step 3 — Guided Swordfish

No identical row patterns.

Learner must:

1. choose target digit;
2. choose three sparse base rows;
3. verify complete source positions;
4. confirm three-cover union;
5. eliminate outside cover candidates.

Step 4 — False size-3 Fish

Show three candidate-sparse rows whose column union contains four columns.

Prompt:

Swordfish?

Correct: No.

Feedback:

Three bases need to be covered by exactly three cover units for a basic size-3 Fish.

Step 5 — Column form

Transpose the logic.

Learner identifies:

• three base columns;
• three cover rows.

Practice

Include:

• 2/2/2 source candidate counts;
• 2/3/2 distribution;
• column-based form;
• false four-cover pattern;
• valid but unproductive pattern.

Hint ladder

1. “Follow one digit.”
2. “Find three sparse rows or columns.”
3. “Merge their opposite-unit positions.”
4. “The union contains exactly three cover units.”
5. Highlight eliminations.

Completion

Three productive Swordfish examples plus one false and one unproductive classification.

Next

XY-Wing

Fish stay on one digit. XY-Wing switches to relationships between bivalue cells.

Reference

Read Swordfish for the irregular Fish proof and comparison with X-Wing.