--- title: "XY-Chain: Link cells with two candidates" description: "An XY-Chain links cells with exactly two candidates each. Follow the example to see where the shared digit at the two ends can be eliminated." lang: "en" canonical: "https://unisu-sudoku.app/en/sudoku-techniques/xy-chain/" --- [Back to the learning guides](https://unisu-sudoku.app/en/learn/) Advanced techniques # XY-Chain: Link cells with two candidates An XY-Chain joins cells with exactly two candidates each. Neighbors see one another and share a candidate. If the chain starts and ends on the same digit, at least one end must contain it. ![Unisu lesson on XY-Chain, showing the explained grid](https://unisu-sudoku.app/_astro/en.CAa-cVPy_2gEA9k.webp) An XY-Chain joins cells with exactly two candidates each. Neighbors see one another and share a candidate. If the chain starts and ends on the same digit, at least one end must contain it. Two cells see each other when they share a row, column or 3×3 box. An elimination needs the specific visibility relationships described in the reasoning. ## An example from the app We connect candidate 9 in A2 to the same digit in D6. The highlighted cells form the route between them. We build the chain one link at a time. A solid line means at least one end must be true. A dashed line means the ends cannot both be true. We will follow the links one at a time. A2 has only 9 and 6. If 9 is false there, 6 must be true. This is a strong link. If 6 is true in A2, 6 is false in C3. These candidates cannot both be true: a weak link. C3 has only 6 and 4. If 6 is false there, 4 must be true. This is a strong link. If 4 is true in C3, 4 is false in C6. These candidates cannot both be true: a weak link. C6 has only 4 and 3. If 4 is false there, 3 must be true. This is a strong link. If 3 is true in C6, 3 is false in D6. These candidates cannot both be true: a weak link. D6 has only 3 and 9. If 3 is false there, 9 must be true. This is a strong link. If 9 were false in A2, it would have to be true at the other end in D6. At least one end must contain it. Cells sharing an area with both ends cannot contain it. These notes are therefore ruled out: D2: 9. The image shows the candidates relevant to this step. A note being hidden does not mean it has been logically eliminated. ## Practice the next step Remove that digit only from cells seeing both ends. Every link must follow from the actual candidates. In the app, follow the reasoning in the matching lesson and try a task yourself. With a new puzzle, always start from its actual candidate state. ## Questions about XY-Chain ### Which cells can belong to an XY-Chain? In this form, cells with exactly two candidates each. Links must follow actual shared areas. ### Why switch candidates inside a cell? With exactly two candidates, one being false makes the other true. That relationship carries the reasoning to the next link. ### Must the same digit appear at both ends? Yes, for the elimination explained here. At least one end cell must contain that shared end digit. ### Must a target see the entire chain? No. It must see both end cells to exclude their shared end digit. ### Does a drawn line prove a link? No. Check each link’s candidates and shared areas. A drawing’s shape or proximity is not logical evidence. ## Keep practicing in Unisu Download Unisu for iPhone or Android. Solve a puzzle or work through a guided lesson on the technique. - [Download on the App Store](https://apps.apple.com/us/app/id6775824061?l=en) - [Get it on Google Play](https://play.google.com/store/apps/details?id=app.unisu.sudoku&hl=en) ## Useful before this guide ### XY-Wing [Read more: XY-Wing](https://unisu-sudoku.app/en/sudoku-techniques/xy-wing/) ### W-Wing [Read more: W-Wing](https://unisu-sudoku.app/en/sudoku-techniques/w-wing/) ## Where to go next Master class Unisu · Oct 27, 2026 ### [AIC: Alternate strong and weak links](https://unisu-sudoku.app/en/sudoku-techniques/aic/) An AIC connects candidates using alternating strong and weak relationships. An assumption can then be followed along the chain to a certain deduction. Unisu ### [Learn Sudoku: a path from rules to techniques – Unisu](https://unisu-sudoku.app/en/learn/) Start with the rules, get comfortable with pencil marks and choose a technique for the situation you’re working on. Download app ## Ready for a puzzle? Take a moment. Download Unisu on your phone. Start a Sudoku or choose a lesson on the technique you want to understand. - [Download on the App Store](https://apps.apple.com/us/app/id6775824061?l=en) - [Get it on Google Play](https://play.google.com/store/apps/details?id=app.unisu.sudoku&hl=en) ![The daily challenge in Unisu](https://unisu-sudoku.app/_astro/en.C-sYR9hJ_1QwI9G.webp) ![A classic Sudoku game in Unisu](https://unisu-sudoku.app/_astro/en.C6DJ1kiE_2dRWDN.webp)