Turbot Fish Sudoku: A Practical Guide to the Four-Candidate Chain

Turbot Fish Sudoku is really a short X-chain

Turbot Fish Sudoku sounds like a fish pattern, but the useful way to understand it is as a four-candidate chain for one digit. Pick a candidate, such as 7, and find two strong links. A strong link means that within a row, column, or box, 7 has only two possible positions, so if one is false the other must be true. Connect one end of the first strong link to one end of the second with a weak link, meaning those two candidates see each other and cannot both be true. The two unconnected ends become the endpoints of the chain. At least one endpoint must be true. Therefore, any outside candidate 7 that sees both endpoints can be eliminated. This compact pattern is closely related to Skyscraper, Two-String Kite, and Empty Rectangle logic, but Turbot Fish Sudoku gives you one chain framework for recognizing the shared idea.

Read Turbot Fish Sudoku from endpoint to endpoint

The cleanest proof for Turbot Fish Sudoku is to test one endpoint as false. If endpoint A is false, its strong-link partner must be true. The weak link then forces the middle candidate on the other side to be false, so the final strong link forces endpoint B to be true. If endpoint A is true instead, you already have a true endpoint. Either way, at least one of A or B contains the digit. A target candidate that sees both cannot survive. The important detail is visibility: the target must share a row, column, or box with each endpoint. It does not need to see the two middle candidates. When searching, scan for conjugate pairs first and only then ask whether one candidate from each pair can see the other. Drawing the chain mentally as strong-weak-strong is safer than relying on a fish-like picture.

Practice Turbot Fish Sudoku after simpler single-digit patterns

Turbot Fish Sudoku becomes much easier once strong links feel automatic. Before hunting it, check for ordinary singles, locked candidates, X-Wings, Skyscrapers, and Two-String Kites because they are easier to verify and may remove the same target. If those fail, choose one digit with several conjugate pairs and trace short connections. Keep the chain on one digit from start to finish. Shapedoku uses symbols instead of numbers, but a crystal candidate can form the same strong and weak links as a 7. On Extreme boards, Notes make those remaining symbol positions visible, so the technique transfers without any arithmetic. If you enjoy following linked consequences through a board, Rune Flow offers a calmer literal connection puzzle where matching crystals are joined by paths: [https://play.google.com/store/apps/details?id=com.snailpixel.runeflow](https://play.google.com/store/apps/details?id=com.snailpixel.runeflow). In Sudoku, though, stop as soon as the endpoint logic proves one clean elimination.

A good Turbot Fish Sudoku practice drill is to ignore elimination targets at first and search only for strong-weak-strong chains. Pick one digit, mark every conjugate pair, and connect pairs whose inner endpoints see each other. Then look at the two outer endpoints and ask which cells see both. This order reduces false positives because the chain is proven before a tempting target enters the picture. It also reveals when a supposed Turbot Fish is really a simpler Skyscraper or Two-String Kite. The name matters less than recognizing that the two endpoints cannot both be false.

1. Choose one digit or symbol and mark its strong links in rows, columns, and boxes.
2. Find two strong links whose inner candidates can see each other through a weak link.
3. Treat the two outer candidates as the endpoints of a strong-weak-strong chain.
4. Confirm that the proposed elimination cell sees both endpoints.
5. Remove only the chained digit from that target cell.
6. After the elimination, return to ordinary singles and locked candidates before searching for another chain.