BUG and BUG+1
Learn BUG+1: when every unsolved cell but one is bi-value, the single three-candidate cell must take the digit that appears three times in its units.
BUG stands for Bivalue Universal Grave — a board state where every unsolved cell has exactly two candidates and every remaining digit appears exactly twice in every unit. Such a grid would have two solutions, so a unique puzzle can never actually reach it.
BUG+1 is the position one step away: every unsolved cell is bi-value except a single cell with three candidates. To avoid sinking into the deadly all-bi-value grave, that cell must take the candidate that appears three times in its row, column and box — place it and the board resolves.
It is a quietly powerful endgame technique, turning a whole-board property into a single placement. The example highlights the lone three-candidate cell and the digit BUG+1 forces into it.
Practise the BUG+1
The best way to learn a technique is to use it. Play a puzzle at the level where it first appears, or drop a tricky board into the solver to watch it in action.
Frequently asked questions
What is BUG+1 in Sudoku?
A near-deadly state where all unsolved cells are bi-value except one with three candidates. The extra cell must take the digit that appears three times in its units, avoiding a multi-solution grid.
When can I use BUG+1?
Late in an expert solve, when the grid has reduced to all bi-value cells plus exactly one trivalue cell. Check that the rest of the board is truly all pairs first.
Does BUG+1 assume a unique solution?
Yes. Like the Unique Rectangle, it relies on the puzzle having a single solution, so it is only valid on properly-set boards.
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