Peak Sudoku
Fish, wings, chains & uniquenessPeak Score 65–75

BUG+1

All unsolved cells are bivalue except one – that cell's odd digit is forced.

Also known as: Bivalue Universal Grave + 1, BUG Type 1

Quick Summary

When every unsolved cell in the entire grid has exactly two candidates except for one single cell that has three candidates, that three-candidate cell must take the digit that appears 3 times in its houses.

Understanding the BUG+1 Technique

BUG stands for Bivalue Universal Grave. If every unsolved cell in a Sudoku grid has exactly two candidates, and every digit appears exactly twice in every row, column, and box, the grid has two solutions—a fatal flaw. If you find a grid where every remaining cell is bivalue EXCEPT for one cell that contains three candidates, that cell must break the deadly pattern. It immediately takes whichever candidate appears an odd number of times (three times) across its containing row, column, and box.

Visual Examples

Green cells highlight the BUG+1 pattern, amber cells denote where candidates are affected, and struck-through pencil marks show the eliminations. Use the buttons or arrow keys to browse examples.

Example 1 of 5
Every empty cell has two candidates except R2C3, which has three: {2,8,9}. If it dropped to two as well, the puzzle would have a second solution. Only 9 appears an odd number of times across R2C3's row, column, and box, so it must be 9.
Places 9 in R2C3

How to Spot & Solve a BUG+1 (Step-by-Step Guide)

The complete visual scanning and logical deduction process to execute this technique:

  • 1Scan the board when almost all unsolved cells have been reduced to exactly two pencil marks.
  • 2Identify the single cell on the board that has three candidates instead of two.
  • 3Count how often each of its three candidates appears across its containing row, column, and 3×3 box.
  • 4Notice that two of the candidates appear twice in those units, while one candidate appears three times.
  • 5To prevent an invalid puzzle state with multiple solutions, the cell must take the candidate with the odd count.
  • 6Directly place that odd-count number into the cell to break the deadlock.

Common Traps & Mistakes to Avoid

  • Using BUG+1 when there are two or more cells with three candidates (that requires BUG+2 or standard chains).
  • Miscounting the candidate occurrences in the containing row, column, or box.
  • Applying BUG logic on an improperly penciled board with missing candidates.

Frequently Asked Questions

Does BUG+1 place a digit immediately?

Yes! BUG+1 is unique among advanced techniques because it directly places a number into the cell rather than merely eliminating candidates.

Expand your repertoire by learning prerequisites and advanced counterparts:

Unique RectanglePeak Score 55–70

A pattern that would create two solutions – logic forces it to break.

Read guide →
XY-ChainPeak Score 70–85

A chain of bivalue cells that forces a digit at both endpoints.

Read guide →

Ready to Practice BUG+1?

Try a puzzle in the Tough tier (Peak Score 61–70), or browse all 25 techniques.

Play Tough puzzleAll 25 techniques