{"id":706,"date":"2025-12-18T17:10:40","date_gmt":"2025-12-18T04:10:40","guid":{"rendered":"https:\/\/thesudoku.com\/blog\/?page_id=706"},"modified":"2026-08-20T16:19:01","modified_gmt":"2026-08-20T04:19:01","slug":"bi-value-universal-grave","status":"publish","type":"page","link":"https:\/\/thesudoku.com\/blog\/bi-value-universal-grave\/","title":{"rendered":"Bi-value Universal Grave"},"content":{"rendered":"\n
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< <\/a>Back<\/a><\/p>\n<\/div>\n\n\n\n

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How to Master the Bi-value Universal Grave (BUG+1) Strategy in Sudoku<\/h1>\n\n\n\n

When an Expert or Master Sudoku puzzle is almost solved, you might reach a state where nearly every empty box holds exactly two candidates. Before you start guessing, check the board for a Bi-value Universal Grave (BUG)<\/strong>. This technique looks at the global uniqueness of the entire grid to give you an instant single-move solution.<\/p>\n\n\n\n

What is a Bi-value Universal Grave?<\/strong><\/h2>\n\n\n\n

A Bi-value Universal Grave (BUG) is a grid state where every unsolved cell contains exactly two candidates, and no Hidden Singles are present. This is a type of Deadly Pattern that cannot appear in a valid Sudoku puzzle.<\/p>\n\n\n\n

If a Sudoku grid ever reached a full BUG state, every row, column, and block would have every remaining candidate appear exactly twice<\/strong>. This would make the entire grid completely symmetrical and unresolvable, yielding at least two valid solutions and breaking the fundamental rule of Sudoku uniqueness.<\/p>\n\n\n\n

We should avoid this deadly pattern from appearing in our Sudoku.<\/p>\n\n\n\n

Identifying BUG+1 (The Savior Cell)<\/strong><\/h2>\n\n\n\n

In actual gameplay, a puzzle will never devolve into a full BUG. Instead, you will encounter a BUG+1<\/strong> pattern:<\/p>\n\n\n\n

Consider this example: All of these cells, except the highlighted one, have exactly two candidates left, while the highlighted cell has three candidates.<\/p>\n\n\n\n

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