Finned Swordfish Sudoku: How to Find and Verify the Pattern

Finned Swordfish Sudoku builds on three base and three cover lines

Finned Swordfish Sudoku begins with the ordinary Swordfish idea. For one digit, three base rows constrain all of that digit's relevant candidates to three cover columns, or the orientation can be reversed. In a clean Swordfish, those three required placements occupy the three cover lines, so other candidates for the digit can be removed from the covers. A finned version has one or more extra candidates in a base house outside the clean cover structure. Those extras are fins. They stop you from making the full Swordfish elimination, but they can still allow a local elimination near the fin. The geometry is more demanding than a Finned X-Wing because three base houses are involved. Start by identifying the nearly complete Swordfish first. If you cannot state the three bases and three covers clearly, do not label the pattern yet.

Prove Finned Swordfish Sudoku with the fin true or false

The logic of Finned Swordfish Sudoku is conditional. If a fin is true, the digit is excluded from peers of that fin, especially inside its box. If the fin is false, the remaining candidates collapse into the ordinary Swordfish and the relevant cover lines provide the elimination. A target is valid only when it is removed in both branches. In practice, that usually means the target sits in a cover line and also sees every fin through a shared box. This is much narrower than the elimination area of an unfinned Swordfish. Multiple fins require extra care: the target must be protected against every way a fin can satisfy the base house. A useful checking habit is to circle the fin box, mark the cover line through the target, and verify that no unaccounted candidate in the base sets escapes the three-cover structure.

Search for Finned Swordfish Sudoku only after simpler fish

Finned Swordfish Sudoku is rarely the first advanced pattern worth checking. Scan for X-Wings, Finned X-Wings, Swordfish, Skyscrapers, and other short single-digit chains first. If one digit has a dense but orderly candidate map, then inspect groups of three rows or columns for an almost-Swordfish whose extra candidates cluster in one box. Full Notes are essential because a forgotten candidate can invalidate the entire pattern. In Shapedoku, the same fish logic works with a shape such as the lotus because the symbol only needs to appear once in every row, column, and box. On Extreme difficulty, use Notes to map one shape at a time rather than trying to see all nine candidate networks at once. After Finned Swordfish Sudoku removes a candidate, stop and rescan for a simpler consequence; one advanced elimination often unlocks a basic single.

To make Finned Swordfish Sudoku manageable on paper, write the three base houses in the margin and list the cover columns or rows beside them. Then note the fin box separately. This turns a sprawling visual pattern into a compact checklist. If a candidate appears in one base house outside both the three covers and the recognized fin area, stop: the pattern is not the fish you thought it was. Likewise, if the proposed target does not see every fin, the two-case proof fails. A few seconds of bookkeeping can prevent an elimination that would corrupt the whole candidate map.

1. Pick one digit and choose either rows as bases or columns as bases.
2. Find three base houses whose candidates nearly fit inside three cover houses.
3. Identify extra base candidates outside the clean cover structure as fins.
4. Check that the target lies on a relevant cover and sees all fins through a shared unit.
5. Verify the target disappears whether a fin is true or all fins are false.
6. Return to ordinary solving immediately after the elimination.