Turbot Fish

Updated 15 October 2026

A Turbot Fish is a four-cell chain on one digit: a strong link, a weak link, and a second strong link, where each strong link can come from a row, a column or a box. One end of the chain always holds the digit, so any cell that sees both ends cannot.

How Suuudokuuu finds a Turbot Fish

The Turbot Fish is the shortest X-Chain there is, and Suuudokuuu finds it with the X-Chain detector rather than a detector of its own. That detector searches shortest chains first, so four-cell chains like this one are what it reaches for before anything longer, and the hint and the solver report them as X-Chains. The Skyscraper and the 2-String Kite are the two best-known Turbot Fish shapes; the example below is neither, because one of its strong links lives in a box.

The logic does not care which kind of unit each link uses. A strong link only says that if one cell is not the digit, the other is. The weak link says the two middle cells cannot both be the digit. Chain them and one end is always forced: if the first end is empty of the digit, the strong link puts it in the second cell, the weak link clears the third, and the second strong link puts it in the fourth.

Worked example

The first strong link is row 3, where 9 fits only r3c1 and r3c3. The weak link runs down column 3 from r3c3 to r9c3. The second strong link is box 7, where 9 fits only r9c3 and r9c1 — r9c3 and r9c1 also share row 9, but it is the box that confines the digit to those two cells. The chain’s ends, r3c1 and r9c1, both sit in column 1, so every other cell of column 1 that still carries 9 loses it: r4c1 and r5c1.

How to spot a Turbot Fish

1. List the strong links for one digitFor one digit, mark every row, column and box where it has exactly two candidates.
2. Join two strong links with a weak linkFind two of those links where a cell of one sees a cell of the other through a shared row, column or box.
3. Erase the digit from cells seeing both endsThe two cells left over are the chain’s ends. Remove the digit from every cell, outside the chain, that sees both.

Common mistakes

• Ignoring box links. A box with only two candidates for the digit is as good a strong link as a row or a column.
• Joining two strong links that share a cell. Then there is no weak link in the middle, only one longer strong chain.
• Forgetting the cells in the ends’ own unit. When both ends share a line, the whole rest of that line is cleared.

Long runs of strong links alone are handled by Simple Coloring, and longer alternating chains by the X-Chain itself. Every pattern the engine implements is on the technique index.