--- title: "Locked Candidates: Use the overlap between a box and a line" description: "Locked Candidates: when all candidates for a digit in a box lie on one row or column, remove that digit from the rest of that line outside the box." lang: "en" canonical: "https://unisu-sudoku.app/en/sudoku-techniques/locked-candidates/" --- [Back to the learning guides](https://unisu-sudoku.app/en/learn/) Core techniques # Locked Candidates: Use the overlap between a box and a line If a digit’s candidates in a box all lie in one row or column, that digit cannot appear elsewhere on that line outside the box. This is one form of Locked Candidates. ![Unisu lesson on Locked Candidates, showing the explained grid](https://unisu-sudoku.app/_astro/en.ClIyyn2e_1SOcpS.webp) If a digit’s candidates in a box all lie in one row or column, that digit cannot appear elsewhere on that line outside the box. This is one form of Locked Candidates. ## An example from the app In box 4, 4 fits only in A4, C4. Those cells all lie in row 4. The box’s 4 must therefore be in row 4. It cannot appear again on that line outside the box. This is Pointing: from box to row or column. These notes are therefore ruled out: G4: 4. 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 Check every possible position of the digit in the box. The reverse direction, from line to box, is another variation you can practice in the app. 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 Locked Candidates ### Which areas does Locked Candidates connect? A row or column and a 3×3 box. One area restricts all positions of a digit to their shared line. ### Must there be exactly two candidates? No. What matters is the restriction of every position of that digit, rather than a fixed number of visible notes. ### Where can I eliminate the digit? From box to line, eliminate outside the box on that line. From line to box, eliminate inside the box outside the starting line. ### Are two candidates next to each other enough? No. The starting area must have no other position for that digit outside the shared line. ### Does the method work with columns? Yes. The box–column relationship follows the same reasoning as the box–row relationship. ## 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 ### Hidden Single [Read more: Hidden Single](https://unisu-sudoku.app/en/sudoku-techniques/hidden-single/) ### Sudoku pencil marks: track and update candidates [Read more: Sudoku pencil marks: track and update candidates](https://unisu-sudoku.app/en/sudoku-techniques/pencil-marks/) ## Where to go next Further techniques Unisu · Oct 27, 2026 ### [X-Wing: Two rows, the same two columns](https://unisu-sudoku.app/en/sudoku-techniques/x-wing/) An X-Wing has a digit in exactly two possible cells in each of two rows, in the same two columns. Learn which candidates this pattern rules out. 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)