--- title: "ALS-XY-Wing: Sudoku Technique #34 | Sudoku Flow" description: "ALS-XY-Wing: Two almost-locked wing sets joined through a hinge by two different restricted common candidates force a third shared candidate into one of the…" url: https://sudokuflow.com/lessons/alsXYWing/ language: en --- [Sudoku technique lessons](https://sudokuflow.com/lessons/) / ALS-XY-Wing Lesson 34 # ALS-XY-Wing ## The rule Two almost-locked wing sets joined through a hinge by two different restricted common candidates force a third shared candidate into one of the wings, so that candidate can be removed from any outside cell that sees every cell with that candidate across both wings. ## Step by step 1. Look at the first wing . These 3 cells all lie in the same row and have 4 different candidates among them, one more than the number of cells. Such a set is called an Almost Locked Set (ALS): remove one of its candidates and it locks, placing every remaining candidate inside itself. 2. The second wing is an ALS too: these 3 cells all lie in the same column and have 4 different candidates among them, one more than the number of cells. 3. The hinge set is also an ALS, and it links the two wings. With the first wing it shares candidate 1 as a restricted common candidate (RCC): every cell holding 1 in one set sees every cell holding 1 in the other, so the digit 1 can serve at most one of the two sets. With the second wing it shares candidate 7 the same way. 4. Could a cell that sees every 9 in both wings still be 9? No: it would strip 9 from both wings and lock them. The first wing would then use 1 and the second 7, and through the restricted links the hinge would lose both 1 and 7, leaving it fewer candidates than cells. So 9 must be placed inside one of the two wings, and every outside cell that sees all the 9 candidates of both wings loses 9. That is where the name comes from: X and Y are 1 and 7, the candidates linking the hinge to each wing, and Z is 9, the candidate removed. [Read the full lesson, with more examples →](https://sudoku.rubigogames.com/techniques/als-xy-wing.html) In our app, when this technique is the next step, the hint names it and shows you why, step by step. No ads, free to download. [App Store iPhone, iPad, Mac](https://apps.apple.com/app/apple-store/id6502922368?pt=125379530&ct=sudokuflow_cta&mt=8) [Google Play Android](https://play.google.com/store/apps/details?id=com.rubigogames.sudoku&referrer=utm_source%3Dsudokuflow%26utm_medium%3Dcta%26utm_campaign%3Dlesson) ## Try it on more boards Each board is a real puzzle from our app, at a moment where ALS-XY-Wing applies. Find the step yourself, or tap Hint and the app's own hint walks you through it. ALS-XY-Wing practice board: ```text 4 . . | . . 8 | . 7 6 . 8* . | 6 . . | 4* 2* . . 7 6 | 4* . 2* | . 8 1 ---------+----------+--------- 2* . 9 | 7 . . | 8 1* 3* . . . | 3 8* . | . 6 2 8 6* 3* | . 2 1 | 7* 4* . ---------+----------+--------- 5* 9 4* | 2* . . | 1 3 8* . . . | 8 . . | 6* 5* 4* 6* 3* 8 | 1* 4 5 | 2 9* 7* ``` Digits marked * were filled in by a player; the others were printed with the puzzle. ![Sudoku app icon](https://sudokuflow.com/assets/app-icon.png) ## Want more boards like this? We also make a Sudoku app for iPhone, iPad, Mac and Android. There are no ads in it at all, it works offline, and when you ask for a hint it shows you the technique instead of just filling in a number. [App Store iPhone, iPad, Mac](https://apps.apple.com/app/apple-store/id6502922368?pt=125379530&ct=sudokuflow_web&mt=8) [Google Play Android](https://play.google.com/store/apps/details?id=com.rubigogames.sudoku&referrer=utm_source%3Dsudokuflow%26utm_medium%3Dweb%26utm_campaign%3Dsite) Sudoku Flow is made by Rubigo Games, the developer of the Sudoku app recommended on this site.