The Finned X-Wing Sudoku Technique Explained

Begin with the ordinary X-Wing idea

For one symbol, an X-Wing uses two rows where that symbol appears in exactly the same two columns. The symbol must occupy opposite corners of the rectangle, so other candidates for that symbol can be removed from those two columns. A Finned X-Wing starts with a similar rectangle, but one of the rows has an extra candidate close to one corner. That extra candidate is the fin. Because of it, you cannot make the full column eliminations of a normal X-Wing. The fin creates an exception, but it also creates a smaller, local elimination that is still logically sound. It helps to think of the extra candidate as preventing the global fish while preserving pressure inside one box. The pattern is therefore not a broken X-Wing. It is a different conditional structure with a more limited target area.

Find the base, cover, and fin

Choose a symbol and look for two base rows. One row should have exactly two candidates for the symbol. The other should have candidates in the same two cover columns plus one extra candidate. The extra candidate must share a box with one of the rectangle corners in its row. That nearby corner is often called the finned corner. The geometry matters. If the extra candidate sits in a different box from both aligned corners, the pattern does not produce the standard Finned X-Wing elimination. You can transpose the whole idea and use two base columns with two cover rows instead. Some solvers call the row with two candidates the clean base and the row with the extra candidate the finned base. Terminology varies, so focus on the candidate layout. Sketching the rectangle lightly on paper or following highlighted shapes on screen can make the geometry easier to confirm.

Understand the local elimination

Consider the two possibilities. If the fin is false, the remaining four rectangle positions behave like a normal X-Wing, so the relevant cover column must contain the symbol at one of its corners. If the fin is true, it directly claims the symbol in its box. In either case, any candidate that sees both the fin and the aligned rectangle corner cannot be true. The eliminations therefore occur inside the fin's box, along the cover column or row shared with that corner. This is narrower than an X-Wing elimination, but it can be enough to create a single and restart the puzzle. Be conservative about targets. A candidate outside the fin's box usually does not see the fin and therefore cannot be removed by this pattern. After the elimination, return to ordinary scanning. The value of a Finned X-Wing often lies in one small removal that creates a hidden single or locked candidate.

1. Pick one shape and map its candidate positions across every row.
2. Find two rows that nearly form an X-Wing on the same two columns.
3. Confirm that one row has one extra candidate, the fin.
4. Check that the fin shares a box with one aligned rectangle corner.
5. Remove the shape only from cells that see both the fin and that aligned corner.
6. Repeat the logic with rows and columns swapped when needed.