Turbot Fish

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A Turbot Fish is a same-digit two strong link chain. One strong link comes from a conjugate pair in a box, and the other comes from a conjugate pair in a row or a column. Unlike a Skyscraper or a Two-String Kite, the two strong links do not have a very obvious geometric relationship.

For a given candidate digit, if you can find one conjugate-pair strong link in a box and another in a row or a column, and choose one node from each so that those two chosen nodes lie in the same column or row and therefore form a weak link, then the pattern is a Turbot Fish. The two unchosen nodes are the endpoints of the chain. Any candidate that forms weak links with both endpoints can be eliminated.

For example, look at digit 3 in the board below. There is one conjugate pair in row 3: 3 appears only in R3C2 and R3C9. There is another conjugate pair in box 6: 3 appears only in R8C3 and R9C2. These form two strong links. At the same time, R3C2 and R9C2 lie in the same column, so they form a weak link. This gives a two-strong-link chain with endpoints R3C9 and R8C3. By alternating-inference-chain logic, one of those endpoints must be true. Candidate 3 in R8C9 forms weak links with both endpoints, so it can be eliminated.

The characteristic feature of a Turbot Fish is the conjugate pair cramped inside a box, which resembles the two close-set eyes of a flatfish. Here are some other forms of the pattern.