Swordfish

A Swordfish is a size-3 basic Fish.

For one candidate digit, three source rows restrict all of their possible placements to the same three columns. Those three rows need three copies of the digit, so the three cover columns must receive those copies from the source rows.

Candidate copies of the digit can therefore be removed from the cover columns outside the three base rows.

Rows and columns can swap roles.

The key difference from X-Wing is not a new proof.

It is a bigger set:

3 base sets → 3 cover sets.

Quick rule

For a row-based Swordfish:

1. Choose digit X.
2. Select three rows whose candidate X positions are all contained in the same three columns.
3. The rows are the base sets.
4. The columns are the cover sets.
5. Remove X from the cover columns outside the base rows.

A base row may contain two or three source candidates.

The three rows do not need identical candidate-position patterns.

Why Swordfish works

Each base row must eventually contain one copy of the Fish digit.

Three base rows therefore require three placements.

All source candidates are covered by only three columns.

Those three placements must occupy those three columns, one per column.

Any additional candidate X in the cover columns outside the base rows would create an impossible extra copy.

This is exactly the X-Wing counting proof with N = 3 instead of N = 2.

A Swordfish does not need a 3×3 candidate grid

Consider:

• row 1 → candidate 2 at columns 1 and 8;
• row 4 → candidate 2 at columns 1, 5, and 8;
• row 9 → candidate 2 at columns 5 and 8.

The union of source columns is:

{c1, c5, c8}

Three rows.

Three cover columns.

That can form a Swordfish.

Several row/column intersections contain no source candidate at all.

The Fish still works.

Step-by-step board example

Column-based Swordfish

Transpose the pattern.

Three columns may restrict candidate 6 to exactly three rows.

Then:

• columns = bases;
• rows = covers.

Candidate 6 can be removed from those cover rows outside the base columns.

Swordfish vs X-Wing

The strongest learning connection is:

X-Wing
2 bases / 2 covers

Swordfish
3 bases / 3 covers

But recognition differs.

X-Wing often shows two identical position pairs.

Swordfish frequently uses:

• different two-position pairs;
• one or more three-position base rows;
• an irregular visual shape.

That is why Swordfish should be learned by counting cover units, not by searching for a particular picture.

How to search for Swordfish

A manual solver should avoid testing arbitrary triples of rows.

Use sparse candidate counts.

1. Filter one digit

2. Find rows containing two or three candidates

3. Compare small groups

Take three promising rows and count the union of their candidate columns.

4. Look for exactly three covers

If the union is three columns, verify the complete source state.

5. Look for outside candidates in the cover columns

If none exist, the Fish is valid but unproductive.

6. Repeat vertically if needed

Swordfish vs Naked Triple

Both use “three things covered by three things,” but the objects differ.

Naked Triple

3 cells reserve 3 candidate digits inside one unit.

Swordfish

3 base units require 3 placements covered by 3 opposite units for one digit.

This comparison exposes the broader Sudoku habit of counting constrained sets.

Swordfish vs Jellyfish

Jellyfish is the size-4 version.

VeyraPlay keeps it in a later Expert expansion because the current learning value is better served by mastering the base/cover concept with X-Wing and Swordfish before expanding the same proof again.

What Swordfish usually unlocks

Swordfish
↓
candidate removed from cover column
↓
Hidden Single
↓
box restriction
↓
Locked Candidates

Make the advanced elimination, then return to simpler logic.

Common mistakes

Requiring three candidates in every base row

Two or three source positions are normal.

Requiring identical candidate columns in all three rows

Not required. The union of source cover columns must contain exactly three columns.

Accepting four cover columns

Then you do not have a size-3 basic Fish.

Eliminating source candidates

Keep base candidates. Remove the digit from cover-set cells outside the bases.

Mixing candidate digits

One Fish = one digit.

Recognition drill

Write sparse candidate maps:

r1 → c1/c8
r4 → c1/c5/c8
r9 → c5/c8

Then ask:

How many distinct cover columns exist across these three rows?

If the answer is three, inspect for Swordfish eliminations.

FAQ

Does Swordfish require nine source candidates?

No.

Can one base row contain two candidates?

Yes.

Is Swordfish just a larger X-Wing?

It uses the same basic Fish proof at size 3.

Should I learn Jellyfish next?

Not necessarily. For the current learning path, moving into Wings and Links gives more variety and value than immediately scaling Fish again.

What to learn next

Move to XY-Wing to switch from one-digit unit patterns to bivalue-cell relationships.