Empty Rectangle
The Empty Rectangle is a single-digit solving technique which uses the absence of candidates to perform an elimination.
An alternative term is hinge, which was actually coined before the term Empty Rectangle, but to a smaller audience. The acronym ER is used by many players.
How it works
Take a look at this diagram:
Row 2 has a strong linked pair of candidates in cells A and B. Call that candidate x. Box 9 has all candidates x confined to 1 boxcol and 1 boxrow. The 4 cells without candidate x form the Empty Rectangle (marked with a dash). When x is true in A, x is false in r8c2. When x is false in A, it is true in B, and hence the remaining candidates x in box 9 are confined to row 8, causing x to be false in r8c2. Thus x can never be true in r8c2, and so can be eliminated from r8c2.
This technique is a special case of Grouped Turbot Fish.
Sudoku Games
- Sudoku 9981
- Samurai Sudoku
- Sudoku Printer
- Word Sudoku to Learn English
Word Sudoku to Learn Chinese
Word Sudoku to Learn French
Word Sudoku to Learn German
Word Sudoku to Learn Spanish
Sudoku strategies
- Coloring
- Singles
- Fish
- Chains and Loops
- Intersections
- Subsets
- Wings
- Single Digit Patterns
- Uniqueness
- Almost Locked Sets
