--- title: "BUG+1: Sudoku Technique #28 | Sudoku Flow" description: "BUG+1: In a puzzle with a unique solution, suppose every unsolved cell has two candidates except one cell with three. If removing one candidate from that cell…" url: https://sudokuflow.com/lessons/bugPlusOne/ language: en --- [Sudoku technique lessons](https://sudokuflow.com/lessons/) / BUG+1 Lesson 28 # BUG+1 ## The rule In a puzzle with a unique solution, suppose every unsolved cell has two candidates except one cell with three. If removing one candidate from that cell would leave every digit appearing as a candidate either zero or two times in each row, column and box, that candidate must be true: otherwise, any solution could be mirrored by swapping each unsolved cell to its other candidate. ## Step by step 1. Every unsolved cell left on the board is bivalue: it holds exactly two candidates. The only exception is the highlighted cell, which holds three: 2, 4, 6. 2. Now look at the highlighted cell's row, column and box: the candidate 6 appears three times in each of them, while every other candidate appears exactly twice, if at all. If this cell were not 6, every candidate would appear exactly twice everywhere, a deadly pattern that could be completed in two different ways. The puzzle has only one solution, so 6 must go here, and 2 and 4 can be removed. [Read the full lesson, with more examples →](https://sudoku.rubigogames.com/techniques/bug-1.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 BUG+1 applies. Find the step yourself, or tap Hint and the app's own hint walks you through it. BUG+1 practice board: ```text 9 6 2* | 7* 3 8* | 4* 1* 5 . . 4 | 9* 5* 1* | 6* 2* . . 1 . | 2 4* 6* | 9 7* . ---------+----------+--------- 2* . 7 | 6* 8* 4 | . . 1* 6* 5 . | 1 7* 2 | . . 4* 4* 8 1* | 3* 9 5 | 2 6* 7 ---------+----------+--------- 3 . . | 4* . 7* | . . 6* 1 4* 8* | 5 6 9 | 7* 3 2* . . 6* | 8* . 3* | . 4 9 ``` 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.