Unique Rectangle Type 1

Unique Rectangle Type 1 prevents a deadly pattern where four cells with identical candidate pairs would create multiple solutions. When three corners have only the pair and one has extras, the pair can be eliminated from the fourth.

Technique

1. Find four cells forming a rectangle across exactly two boxes.
2. Three cells contain only candidates {X,Y}.
3. The fourth cell contains {X,Y} plus additional candidates.
4. Eliminate X and Y from the fourth cell to prevent the deadly pattern.

Tips

• The rectangle must span exactly two rows, two columns, and two boxes.
• This technique assumes the puzzle has a unique solution.
• If X and Y remained in all four cells, they could be swapped infinitely.