Hidden Quad

A Hidden Quad is the four-cell extension of Hidden Pairs and Triples. When four candidates appear exclusively in four cells within a house, all other candidates can be eliminated from those cells.

Technique

1. Within a house, identify four digits that appear only within the same four cells.
2. These four cells must contain these four digits.
3. Eliminate all other candidates from these four cells.

Tips

• Hidden Quads are quite rare - don't spend too much time looking initially.
• They often appear in puzzles designed to require advanced techniques.
• Software or systematic checking helps identify them reliably.