BUG (Bivalue Universal Gravekard)
A uniqueness shortcut: when every unsolved cell is bi-value except one, the odd candidate is the answer.
BUG — Bivalue Universal Grave — is a late-game uniqueness pattern. If the grid reaches a state where every remaining cell has exactly two candidates except a single cell with three, then placing the "extra" candidate is the only way to keep the solution unique.
A full BUG (all cells bi-value) would mean two solutions, which a valid puzzle cannot have — so the technique breaks that symmetry just before it forms.
How to spot it
Once almost every empty cell is bi-value, look for the lone tri-value cell. The digit that appears an odd number of times in that cell’s row, column, or box is the one to place — it is the candidate whose removal would create the deadly bi-value grave.
- Nearly all empty cells are bi-value.
- Exactly one cell has three candidates.
- Place the candidate that appears an odd number of times in a unit.
Worked example
- Late in an expert grid every empty cell shows two candidates — except one showing {2,5,7}.
- Counting 2s, 5s and 7s in its column, the 7 appears three times (odd), the others twice.
- A bi-value grave would have two solutions.
- To keep uniqueness, the tri-value cell must be 7.
- Place 7 and finish with singles.
Try it yourself
Tap a cell, then a number, to practise.
Frequently asked questions
- When does BUG apply?
- Only very late, when the candidate grid has collapsed almost entirely to bi-value cells.
- Is BUG a uniqueness technique?
- Yes — like unique rectangles, it assumes the puzzle has exactly one solution.
Related techniques
Practice: BUG (Bivalue Universal Gravekard)
Put the BUG (Bivalue Universal Gravekard) to work on a live board — free puzzles with notes, hints and four difficulty levels.
Try it on a live board