A Finned X-Wing is an X-Wing-like Fish with one or more extra source candidates — called fins — outside the two normal cover sets.
Those extra candidates prevent the full eliminations of a Basic X-Wing.
But they do not always destroy the pattern completely.
A more limited elimination can still be valid when a candidate:
- would be eliminated by the underlying X-Wing if every fin were false; and
- can also be eliminated if any fin is true because it sees every relevant fin.
The practical rule is:
Only X-Wing cover targets that see all the fins can be eliminated.
Finned X-Wing is the smallest and most useful introduction to the wider Finned/Sashimi Fish family.
Start from a basic X-Wing
A row-based Basic X-Wing uses:
- two base rows;
- two cover columns;
- every source candidate for the Fish digit in those rows contained inside the two covers.
For example:
r2 → c3/c8
r7 → c3/c8The two required placements in r2/r7 must occupy c3/c8, so candidate X can be removed from those columns outside the base rows.
A Finned X-Wing begins when that clean structure is almost present, but one base contains an extra source candidate outside the two covers.
For example:
r2 → c1/c3/c8
r7 → c3/c8Candidate X at r2c1 is the fin.
Why the basic X-Wing eliminations no longer work
If the fin r2c1 is false, the remaining candidates in r2/r7 form the clean X-Wing on c3/c8.
But if the fin is true, row 2 places X at c1 instead.
That means the ordinary X-Wing proof cannot guarantee every outside elimination in c3/c8.
So a Finned X-Wing is not permission to perform all the eliminations of the almost-X-Wing.
The fin restricts which targets remain provably false.
The two-case proof
A useful Finned X-Wing can be understood by splitting the logic into two cases.
Let F be the fin and T be a proposed elimination target.
Case 1: the fin is false
If F is false, the remaining source candidates form the Basic X-Wing.
The X-Wing eliminates T.
Case 2: the fin is true
If F is true, any candidate T that sees F is impossible because both use the same digit and share a house.
Therefore:
- fin false → X-Wing eliminates T;
- fin true → fin eliminates T.
T is false in both cases.
So T can be eliminated.
With multiple fins, the target must be eliminated in every possible fin-true case. In practice, that means the target must see all relevant fins.
Step-by-step Finned X-Wing example
Where valid elimination targets usually appear
In ordinary 9×9 Sudoku, the fin often lies in a box that intersects one of the X-Wing cover columns.
The useful target is then typically:
- in that same box as the fin;
- on one of the Fish cover columns;
- outside the base row containing the fin.
That geometry lets the target be attacked by both branches:
- the Basic Fish branch if the fin is false;
- the fin itself if the fin is true.
Do not memorize only the picture, though.
The proof is the reusable part:
proposed target must be false whether the fin is false or true.
What is a Sashimi X-Wing?
A Sashimi X-Wing uses the same fin logic, but the Fish that remains when the fins are removed is incomplete or degenerate rather than a full ordinary X-Wing.
Terminology varies across Sudoku communities, but the useful distinction is:
Finned X-Wing
Remove the fin(s), and a complete Basic X-Wing remains.
Sashimi X-Wing
Remove the fin(s), and the remaining size-2 Fish is incomplete/degenerate — often because one expected corner candidate is missing or a simpler forced placement replaces the full Fish shape.
The elimination proof remains case-based.
The target must still be impossible:
- under the no-fin branch;
- and under every fin-true branch.
Do not use the name as the proof
Finned and Sashimi Fish terminology can become visually complicated.
A pattern may also overlap with another single-digit technique depending on how it is described.
For human solving, the safest approach is:
- identify the candidate digit;
- identify the two base sets;
- identify the two intended cover sets;
- identify every source candidate outside those covers as a fin;
- test each proposed target against both branches of the proof.
If the target is not false in every branch, do not eliminate it.
Finned X-Wing vs basic X-Wing
Basic X-Wing
All source candidates in the two bases lie inside the two covers.
Outside cover candidates can be eliminated broadly.
Finned X-Wing
One or more source candidates lie outside the covers.
Only the subset of X-Wing targets that also see every fin remain eliminable.
This is why a Finned Fish is not simply “an X-Wing with an extra candidate.”
The extra candidate changes the elimination region.
Finned X-Wing vs Sashimi X-Wing
Both use fins and the same two-case logic.
The difference concerns the Fish left behind if the fins are false.
- Finned: a complete X-Wing remains.
- Sashimi: the remaining Fish is incomplete/degenerate.
For solving correctness, verifying the elimination is more important than arguing about borderline naming.
Finned X-Wing vs Skyscraper
Both are single-digit patterns and can sometimes appear geometrically related.
A Skyscraper is usually taught through:
- two strong-link-like row or column pairs;
- aligned bases;
- two roof candidates;
- elimination from cells seeing both roofs.
A Finned X-Wing is taught through Fish base/cover sets plus a fin.
Some candidate maps can invite more than one description.
Use the proof that is clearest and do not assume similar-looking patterns have identical target rules.
Finned X-Wing vs larger Finned Fish
The same principle scales.
Finned Swordfish
3 base sets / 3 cover sets + one or more fins.
Finned Jellyfish
4 base sets / 4 cover sets + one or more fins.
The target rule remains stricter than in a Basic Fish:
an elimination must survive every fin possibility.
Larger Finned Fish are harder to recognize manually, so Finned X-Wing is the best place to learn the logic.
Multiple fins
A Fish can have more than one fin.
The key requirement becomes:
a proposed target must see all fins relevant to the pattern.
If target T sees fin F1 but not fin F2, then the branch where F2 is true may leave T legal.
In that case T cannot be eliminated by this Finned Fish proof.
This is a common source of false eliminations.
How to find Finned X-Wings efficiently
Do not scan the entire grid for exotic Fish shapes first.
Start from almost-X-Wings.
1. Filter one candidate digit
Fish are single-digit patterns.
2. Look for two sparse rows or columns
You want a near size-2 Fish.
3. Identify two dominant cover positions
Ask whether most source candidates in the bases align on two opposite houses.
4. Identify the leftover source candidate(s)
Those uncovered base candidates are potential fins.
5. Locate ordinary X-Wing elimination cells
Imagine the fins are false and identify what the underlying Fish would eliminate.
6. Keep only targets that see every fin
Those are the candidates that remain impossible in both branches.
7. Prefer simpler single-digit patterns when available
An Empty Rectangle, Skyscraper, X-Chain, or other shorter proof may explain the same elimination more naturally.
What Finned X-Wing usually unlocks
Like other Fish, Finned X-Wing usually eliminates candidates rather than placing a digit directly.
A typical progression is:
Finned X-Wing
↓
remove one tightly localized cover candidate
↓
Hidden Single / Locked Candidate / subset appears
↓
update candidates
↓
return to simpler logicThe elimination count is often smaller than in a Basic X-Wing, but one precise removal can still break the puzzle open.
Common mistakes
Performing every elimination of the almost-X-Wing
The fin blocks the broad Basic Fish elimination. Keep only targets that also see all fins.
Forgetting a source candidate outside the covers
Every uncovered source candidate matters. Missing one may invalidate the proof.
Eliminating the fin
The fin is a live candidate and one branch of the proof assumes it is true.
Treating Sashimi as a completely different technique
Sashimi uses the same fin logic. The distinction describes the degenerate Fish that remains when fins are removed.
Accepting a target that sees only one of several fins
With multiple fins, the target must survive every branch. Seeing only some fins is not enough.
Mixing candidate digits
One Fish follows one digit.
Pattern-matching without checking both cases
Always verify why the target is impossible whether the fins are false or true.
Recognition drill
Suppose candidate 9 has:
r2 → c1/c4/c8
r7 → c4/c8Treat c4/c8 as the intended X-Wing covers and r2c1 as the fin.
For each candidate 9 outside r2/r7 in c4/c8, ask:
- Would the clean X-Wing eliminate this target if
r2c1were false? - Does this target see the fin
r2c1?
Only a target for which both answers are yes can be eliminated by this single-fin pattern.
FAQ
What is the fin in a Finned X-Wing?
A fin is an extra base candidate for the Fish digit that lies outside the intended cover sets.
Can a Finned X-Wing have more than one fin?
Yes. A valid elimination must then see all relevant fins.
Why can’t I make every normal X-Wing elimination?
Because a true fin can break the ordinary X-Wing placement structure. Only targets contradicted in both the fin-false and fin-true cases remain safe.
Is Sashimi X-Wing the same as Finned X-Wing?
They belong to the same finned Fish family and use the same elimination logic. In Sashimi, the Fish left when the fins are false is incomplete or degenerate rather than a complete Basic X-Wing.
Is a Finned X-Wing harder than Swordfish?
Difficulty depends on the puzzle and recognition, but Finned X-Wing introduces an extra conditional layer beyond Basic Fish, so it is usually treated as an expert technique.
Can Finned X-Wing overlap with other named patterns?
Yes. Advanced single-digit patterns can sometimes support multiple descriptions. Correct inference matters more than the label.
What to learn next
Continue with Empty Rectangle to explore another compact single-digit elimination pattern, then move into Alternating Inference Chains for a more general language that can express many advanced deductions without relying on one fixed geometry.