--- title: "AIC: Alternate strong and weak links" description: "An AIC connects candidates using alternating strong and weak relationships. An assumption can then be followed along the chain to a certain deduction." lang: "en" canonical: "https://unisu-sudoku.app/en/sudoku-techniques/aic/" --- [Back to the learning guides](https://unisu-sudoku.app/en/learn/) Master class # AIC: Alternate strong and weak links An AIC connects candidates using alternating strong and weak relationships. An assumption can then be followed along the chain to a certain deduction. ![Unisu lesson on AIC, showing the explained grid](https://unisu-sudoku.app/_astro/en.DfJsNn0S_Z91XYY.webp) An AIC connects candidates using alternating strong and weak relationships. An assumption can then be followed along the chain to a certain deduction. 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 5 in H2 to the same digit in I4. 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. In row 2, 5 can go only in H2 or I2. If it is false in H2, it must be in I2: a strong link. If 5 is true in I2, 9 is false in I2. These candidates cannot both be true: a weak link. In row 2, 9 can go only in I2 or G2. If it is false in I2, it must be in G2: a strong link. If 9 is true in G2, 8 is false in G2. These candidates cannot both be true: a weak link. In column G, 8 can go only in G2 or G3. If it is false in G2, it must be in G3: a strong link. If 8 is true in G3, 1 is false in G3. These candidates cannot both be true: a weak link. In row 3, 1 can go only in G3 or I3. If it is false in G3, it must be in I3: a strong link. If 1 is true in I3, 1 is false in I4. These candidates cannot both be true: a weak link. I4 has only 1 and 5. If 1 is false there, 5 must be true. This is a strong link. If 5 were false in H2, it would have to be true at the other end in I4. At least one end must contain it. Cells sharing an area with both ends cannot contain it. These notes are therefore ruled out: I2: 5; H6: 5. 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 Here, strong means that if one candidate is false, the other must be true. Weak means they cannot both be true. This example uses an ungrouped chain with the same digit at both ends. 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 AIC ### What is a strong link in AIC? At least one of the two candidates must be true. If one is false, the other follows. ### What is a weak link? Both candidates cannot be true together. One being true therefore makes the other false. ### Can AIC connect different digits? Yes. A chain can change digits within a suitable cell and connect matching candidates through shared areas. ### Is any sequence of highlighted cells a chain? No. Strong and weak relationships must alternate appropriately. Every link needs a verifiable reason. ### Does this elimination work with different end digits? Not by the same rule. The example uses an ungrouped chain with matching end digits; other end conditions need their own reasoning. ## 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-Chain [Read more: XY-Chain](https://unisu-sudoku.app/en/sudoku-techniques/xy-chain/) ### Simple Coloring [Read more: Simple Coloring](https://unisu-sudoku.app/en/sudoku-techniques/simple-coloring/) ## Where to go next Master class Unisu · Oct 27, 2026 ### [ALS: Connect almost locked candidate groups](https://unisu-sudoku.app/en/sudoku-techniques/als/) An ALS is a group of n cells in one area with n+1 distinct digits among their candidates. Two such groups can interact through a restricted common candidate. 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)